Unable for the `MRB_TT_CPTR` object to have a singleton class

Fix typo "overfow" to "overflow" [ci skip]

This object is treated as an immediate value.

Boxing:

* `MRB_NO_BOXING`   (`mrb_value` packed in `struct`)
* `MRB_WORD_BOXING` (`mrb_value` packed in `struct`) default
* `MRB_NAN_BOXING`  (`mrb_value` packed in `double`)

`mrb_int` size

* `MRB_INT32`       (`int32_t` as `mrb_int`)
* `MRB_INT64`       (`int64` as `mrb_int`) conflict with `MRB_NAN_BOXING'

Architecture

* `MRB_64BIT`      (`sizeof(void*)` is 64 bits)
* `MRB_32BIT`      (`sizeof(void*)` is 32 bits)

Improve determining C++ compiler in `tasks/toolchains/gcc.rake`

* Consider CC envvar as C compiler on which to make the decision.
* Consider the case where C compiler is `ccache gcc`, etc.

Refactor `mrblib/mrblib.rake`

Refactor `src/mruby_core.rake`

Merge pull request #5152 from shuujii/move-lib-mruby-core-ext.rb-to-lib-mruby-core_ext.rb-for-consistency

Move `lib/mruby-core-ext.rb` to `lib/mruby/core_ext.rb` for consistency

Allow `bintest` even if build name is not `host`

Fix integer overflow error in mrdb test

Revert "Check if irep->reps is NULL in lv_defined_p"

Check if irep->reps is NULL in lv_defined_p

Use `mrb_int_value` instead of `mrb_fixnum_value` in `src/hash.c`

Remove unused `MRuby::Build#enable_{bin,}test=`

Check if irep->reps is NULL

The writers seem to be unnecessary because `MRuby::Build#enable_{bin,}test`
are used from the beginning.

I misunderstand the meaning of #4483. Sorry.

Fix deprecated `set-env` command error on GitHub Actions [ci skip]

I tried `$GITHUB_PATH` and `$GITHUB_ENV` instead of `set-env`, but
for some reason path was not recognized, so I changed to using
`ACTIONS_ALLOW_UNSECURE_COMMANDS`.

- Remove `mrb_ssize`
- Fix `MRB_FIXNUM_{MIN,MAX}` to 32 bits on `MRB_NAN_BOXING`

In addition, `include/mruby.h` will be scanned for error class symbols.