We don't want to increase number of files that should be compiled by C++
compiler when `enable_cxx_exception` is turned on.

We have introduced following new instructions.

 * `OP_LAMBDA16`
 * `OP_BLOCK16`
 * `OP_METHOD16`
 * `OP_EXEC16`

Each instruction uses 16 bits operand for `reps` index. Since new
instructions are added, `mruby/c` VM should be updated.

Due to new instructions, dump format compatibility is lost, we have
increased `RITE_BINARY_MAJOR_VER`.

In addition, we have decreased the size of `refcnt` in `mrb_irep` from
`uint32_t` to `uint16_t`, which is reasonably big enough.

In addition, stop eager allocation of `mt` table.

Introduced `MRB_PRESYM_INIT_SYMBOLS()`

Use `mrb_open_core()` instead of `mrb_open()`  in `mrbc`

Remove unnecessary `ci0` variables; ref #5272

Remove unnecessary configuration macros; ref #5060

The `init_SYMBOLS()` function implicitly defined in `MRB_PRESYM_DEFINE_VAR_AND_INITER()` requires some familiarity when trying to find it from the caller.
By introducing `MRB_PRESYM_INIT_SYMBOLS()`, it is possible to find directly from the identifier.

The following macros will be removed:
- `ENSURE_STACK_INIT_SIZE`
- `RESCUE_STACK_INIT_SIZE`
- `MRB_ECALL_DEPTH_MAX`

Change `.o.d` back to `.d` because `.pi.d` is no longer created

Build internal mrbc in an internal directory

Fixed `String#unpack` to handle the highest range of integer values

This is a correction based on the review by @matz.
https://github.com/mruby/mruby/pull/5306#pullrequestreview-578378401

Previously, problems occurred when the `fixnum` was exceeded.

- 32-bit cpu mode with `MRB_WORD_BOXING` and `MRB_INT32`:

  ```console
  % bin/mruby -e 'p [0x7fffffff].pack("N").unpack("N")'
  trace (most recent call last):
  -e:1: cannot unpack to Integer: 2147483647 (RangeError)
  ```

- 64-bit cpu mode with `MRB_WORD_BOXING` and `MRB_INT64`:

  ```console
  % bin/mruby -e 'p [0x7fffffff_ffffffff].pack("q").unpack("q")'
  trace (most recent call last):
  -e:1: cannot unpack to Integer: 9223372036854775807 (RangeError)
  ```

Raise SystemStackError if mruby VM stack expansion fails

Check first `0` when converting symbols into strings

This was because it caused `SIGSEGV` when `mruby -v` displayed an unnamed variable.

```console
% bin/mruby -ve 'call { |(a, b)| }'

  ...SNIP...

irep 0x8007d0050 nregs=3 nlocals=1 pools=0 syms=1 reps=1 iseq=12
file: -e
    1 000 OP_LOADSELF   R1
    1 002 OP_BLOCK      R2      I(0:0x8007d00a0)
    1 005 OP_SENDB      R1      :call   0
    1 009 OP_RETURN     R1
    1 011 OP_STOP

irep 0x8007d00a0 nregs=6 nlocals=5 pools=0 syms=0 reps=0 iseq=29
local variable names:
zsh: segmentation fault (core dumped)  bin/mruby -ve 'call { |(a, b)| }'
```

Refine `preprocess_options`; ref d95ffb03

If we modify an option that may have been specified by users, we may
make unintended changes, so it is better not to modify it as much as
possible, IMO.

It's for internal use. Please use `conf.disable_presym`.