To be also able to build mruby without presym in the future. However,
`MRB_QSYM` has been removed and changed as follows:

### Example

|       Type                | Symbol |  Previous Style  |   New Style    |
|---------------------------|--------|------------------|----------------|
| Operator                  | &      | MRB_QSYM(and)    | MRB_OPSYM(and) |
| Class Variable            | @@foo  | MRB_QSYM(00_foo) | MRB_CVSYM(foo) |
| Instance Variable         | @foo   | MRB_QSYM(0_foo)  | MRB_IVSYM(foo) |
| Method with Bang          | foo!   | MRB_QSYM(foo_b)  | MRB_SYM_B(foo) |
| Method with Question mark | foo?   | MRB_QSYM(foo_p)  | MRB_SYM_Q(foo) |
| Mmethod with Equal        | foo=   | MRB_QSYM(foo_e)  | MRB_SYM_E(foo) |

This change makes it possible to define, for example, `MRB_IVSYM(foo)` as
`mrb_intern_lit(mrb, "@" "foo")`, which is useful if we support building
without presym in the future.

Reduce memory usage of Hash object

Fix typo in backward compatible mrb_exc_new_str_lit() macro

## Implementation Summary

* Change entry list from segmented list to flat array.
* Change value of hash bucket from pointer to entry to index of entry list,
  and represent it by variable length bits according to capacity of hash
  buckets.
* Store management information about entry list and hash table to `struct
  RHash` as much as possible.

## Benchmark Summary

Only the results of typical situations on 64-bit Word-boxing are present
here. For more detailed information, including consideration, see below
(although most of the body is written in Japanese).

* https://shuujii.github.io/mruby-hash-benchmark

### Memory Usage

Lower value is better.

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |          344B |          256B |   0.74419x |
|        40 |        1,464B |          840B |   0.57377x |
|       200 |        8,056B |        3,784B |   0.46971x |
|       500 |       17,169B |        9,944B |   0.57949x |

### Performance

Higher value is better.

#### `mrb_hash_set`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  1.41847M i/s |  1.36004M i/s |   0.95881x |
|        40 |  0.39224M i/s |  0.31888M i/s |   0.81296x |
|       200 |  0.03780M i/s |  0.04290M i/s |   1.13494x |
|       500 |  0.01225M i/s |  0.01314M i/s |   1.07275x |

#### `mrb_hash_get`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 26.05920M i/s | 30.19543M i/s |   1.15872x |
|        40 | 44.26420M i/s | 32.75781M i/s |   0.74005x |
|       200 | 44.55171M i/s | 31.56926M i/s |   0.70860x |
|       500 | 39.19250M i/s | 29.73806M i/s |   0.75877x |

#### `mrb_hash_each`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 25.11964M i/s | 30.34167M i/s |   1.20789x |
|        40 | 11.74253M i/s | 13.25539M i/s |   1.12884x |
|       200 |  2.01133M i/s |  2.97214M i/s |   1.47770x |
|       500 |  0.87411M i/s |  1.21178M i/s |   1.38631x |

#### `Hash#[]=`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  0.50095M i/s |  0.56490M i/s |   1.12764x |
|        40 |  0.19132M i/s |  0.18392M i/s |   0.96129x |
|       200 |  0.03624M i/s |  0.03256M i/s |   0.89860x |
|       500 |  0.01527M i/s |  0.01236M i/s |   0.80935x |
#### `Hash#[]`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 11.53211M i/s | 12.78806M i/s |   1.10891x |
|        40 | 15.26920M i/s | 13.37529M i/s |   0.87596x |
|       200 | 15.28550M i/s | 13.36410M i/s |   0.87430x |
|       500 | 14.57695M i/s | 12.75388M i/s |   0.87494x |

#### `Hash#each`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  0.30462M i/s |  0.27080M i/s |   0.88898x |
|        40 |  0.12912M i/s |  0.11704M i/s |   0.90642x |
|       200 |  0.02638M i/s |  0.02402M i/s |   0.91071x |
|       500 |  0.01066M i/s |  0.00959M i/s |   0.89953x |

#### `Hash#delete`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  7.84167M i/s |  6.96419M i/s |   0.88810x |
|        40 |  6.91292M i/s |  7.41427M i/s |   1.07252x |
|       200 |  3.75952M i/s |  7.32080M i/s |   1.94727x |
|       500 |  2.10754M i/s |  7.05963M i/s |   3.34970x |

#### `Hash#shift`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 14.66444M i/s | 13.18876M i/s |   0.89937x |
|        40 | 11.95124M i/s | 11.10420M i/s |   0.92913x |
|       200 |  5.53681M i/s |  7.88155M i/s |   1.42348x |
|       500 |  2.96728M i/s |  5.40405M i/s |   1.82121x |

#### `Hash#dup`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  0.15063M i/s |  5.37889M i/s |  35.71024x |
|        40 |  0.06515M i/s |  3.38196M i/s |  51.91279x |
|       200 |  0.01359M i/s |  1.46538M i/s | 107.84056x |
|       500 |  0.00559M i/s |  0.75411M i/s | 134.88057x |

### Binary Size

Lower value is better.

|    File    |   Baseline    |      New      |  Factor   |
|:-----------|--------------:|--------------:|----------:|
| mruby      |      730,408B |      734,176B |  1.00519x |
| libmruby.a |    1,068,134B |    1,072,846B |  1.00441x |

## Other Fixes

The following issues have also been fixed in the parts where there was some
change this time.

* [Heap use-after-free in `Hash#value?`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-hash-value-md)
* [Heap use-after-free in `ht_hash_equal`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-ht_hash_equal-md)
* [Heap use-after-free in `ht_hash_func`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-ht_hash_func-md)
* [Heap use-after-free in `mrb_hash_merge`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-mrb_hash_merge-md)
* [Self-replacement does not work for `Hash#replace`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-self-replacement-does-not-work-for-hash-replace-md)
* [Repeated deletes and inserts increase memory usage of `Hash`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-repeated-deletes-and-inserts-increase-memory-usage-of-hash-md)
* [`Hash#rehash` does not reindex completely](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-hash-rehash-does-not-reindex-completely-md)
* `mrb_hash_delete_key` does not cause an error for frozen object
* `mrb_hash_new_capa` does not allocate required space first
* [`mrb_os_memsize_of_hash_table` result is incorrect](https://github.com/mruby/mruby/pull/5032#discussion_r457994075)

Define `mrb_value` in struct for Word/NaN boxing

Don't check FD for STDIN/STDOUT/STDERR; #5114

We don't need to require valid STDIN/STDOUT/STDERR. If we require it,
we can't use mruby on an environment that doesn't have valid
STDIN/STDOUT/STDERR such as Windows service process. Windows service
process doesn't have valid STDIN.

Fixed build NaN boxing with 32-bit CPU mode

`SET_CPTR_VALUE()` requires the `p` field on 32-bit CPU mode.

This is to make it possible to distinguish between `mrb_value` and `mrb_sym` in C++ "Function overloading" and C11 "Generic selection".

Suppress `-Wunused-function`

If no gem was specified, it was warning because `init_mrbgems()` was not used.

ref: #4250

That loads 32 bit integer bypassing pool access.

Reduce recompilation when updating presym target files

Fix dependencies of `y.tab.o`

Fix that updating presym target files would recompile almost all files even
if symbols did not change.

In the case of `y.tab.o`, `y.tab.d` was not read, so `y.tab.c` was not
recompiled by updating header files.

It seemed that the cause was overwriting the rake rule, so I deleted
unnecessary settings.

New instructions:
  * OP_LOADL16
  * OP_LOADSYM16
  * OP_STRING16

Size of pools, symbols are `int16_t` but offset representation in the
bytecode was 8 bits. Size of child `irep` array is `int16_t`, too but
this change does not address it.

`minirake` was still used in many occasions.
This reverts commit 3847c7eb.

Since `R-assignment` in CRuby is abandoned.  Single-line pattern matching
in `mruby` only matches single local variable at the moment. Currently
it works as a right assignment to a local variable. It will be enhanced
in the future.

Fix parallel build