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.

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

This reverts commit dc51d89a.

Instead of including `mruby/presym.h` everywhere, we provided the
fallback `mruby/presym.inc` under `include/mruby` directory, and specify
`-I<build-dir>/include` before `-I<top-dir>/include` in `presym.rake`.
So even when someone drops `-I<build-dir>/include` in compiler options,
it just compiles without failure.

When I `#call` the "proc" object created by the `mrb_proc_new_cfunc()` function from Ruby space, the return value did not go into the correct stack position.
This can destroy the calling variable.

This issue is now caused by #5272. sorry.

Addressed an issue where existing programs linking `libmruby.a` could only
be built by adding `<build-dir>/include` to compiler's include path.

This enhances self-containment.

- Changed the `mrb_callinfo::pc` field to point to itself.
  Previously it indicated the return destination of the previous call level.
  `mrb_callinfo::pc` will now hold the address to its own `proc->body.irep->iseq`.
- Removed `mrb_callinfo::err` field.
  This is because `mrb_callinfo::pc - 1` is semantically the same as the previous `err`.
- The `pc0` and `pc_save` variables in `mrb_vm_exec()` are no longer needed and have been deleted.
- It removes the argument because `cipush()` doesn't need to save the previous `pc`.

This enhances self-containment.

Previously `mrb_context::stack` had the current call level stack, but now it owns it.
The `mrb_context::stack` field, which is no longer needed, will be removed.

If there is `env`, `env->c` means `target_class`.

But you still cannot cross C function boundary.

Negative integer `>-65535` had wrong value, e,g, `p(-40550)` printed
`4294926746` since Nov. 2020, sigh.

Follow commit 7150c675 .

Jump target address is `operand (16bit)` + `address of next instruction`.

In addition, `ilen` was made `uint32_t` so that `iseq` length limitation
of 65536 is removed. Only jump target address should be within signed
16bit (-32768 .. 32767).

|        Previous Name         |        New Name         |
|------------------------------|-------------------------|
| MRB_ENABLE_ALL_SYMBOLS       | MRB_USE_ALL_SYMBOLS     |
| MRB_ENABLE_SYMBOLL_ALL       | MRB_USE_ALL_SYMBOLS     |
| MRB_ENABLE_CXX_ABI           | MRB_USE_CXX_ABI         |
| MRB_ENABLE_CXX_EXCEPTION     | MRB_USE_CXX_EXCEPTION   |
| MRB_ENABLE_DEBUG_HOOK        | MRB_USE_DEBUG_HOOK      |
| MRB_DISABLE_DIRECT_THREADING | MRB_NO_DIRECT_THREADING |
| MRB_DISABLE_STDIO            | MRB_NO_STDIO            |
| ENABLE_LINENOISE             | MRB_USE_LINENOISE       |
| ENABLE_READLINE              | MRB_USE_READLINE        |
| DISABLE_MIRB_UNDERSCORE      | MRB_NO_MIRB_UNDERSCORE  |
| DISABLE_GEMS                 | MRB_NO_GEMS             |

* `MRB_ENABLE_SYMBOLL_ALL` seems to be a typo, so it is fixed.
* `MRB_` prefix is added to those without.
* The previous names can also be used for compatibility.

### ASAN report (`MRB_INT32`)

  ```console
  $ bin/mruby -ve '-0x40000000'
  mruby 3.0.0preview (2020-10-16)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_NEGATE:
  00001       NODE_INT 40000000 base 16
  irep 0x6070000001e0 nregs=2 nlocals=1 pools=0 syms=0 reps=0 iseq=9
  file: -e
  /mruby/src/codedump.c:173:49: runtime error: left shift of 49152 by 16 places cannot be represented in type 'int'
  SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /mruby/src/codedump.c:173:49 in
      1 000 OP_LOADI32	R1	-1073741824
      1 006 OP_RETURN	R1
      1 008 OP_STOP

  /mruby/src/vm.c:1138:7: runtime error: left shift of 49152 by 16 places cannot be represented in type 'mrb_int' (aka 'int')
  SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /mruby/src/vm.c:1138:7 in
  ```

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.

That loads 32 bit integer bypassing pool access.

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.

The "a"/"*" specifier of the `mrb_get_args()` function will now return `const mrb_value *`.
This is because it is difficult for the caller to check if it is an array object and write-barrier if necessary.
And it requires calling `mrb_ary_modify()` on the unmodified array object, which is also difficult (this is similar to #5087).

To allow C++ compilation. Fix suggested by @dearblue.

Rename new functions:

- `mrb_convert_type(mrb,val,type,tname,method)`
   => `mrb_type_convert(mrb,val,type,tname,method)`
- `mrb_check_convert_type(mrb,val,type,tname,method)`
   => `mrb_type_convert_check(mrb,val,type,tname,method)`

Old names are defined by macros (support `tname` drop and
`char*` => `mrb_sym` conversion).

The mixture causes warnings on 64 bit Windows (VC).

You can now use `NEXT` within `switch` statement like 7c087eb.

On non-`gcc` compatible environment, `NEXT` is translated to `break`.

To silence some warnings. This change cancels part of 7ef3604134.

In the past code, the current `callinfo (ci)` was modified, thus it was
possible to pop `ci` beyond the `cibase`, that could cause out of memory
bound access for the code like the following:

```ruby
def m2
  lambda {
    Proc.new {
      return :return # return from the method
    }
  }.call.call
  :never_reached
end

p m2
```