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.

- `MRB_64BIT` and `MRB_INT32`
- `MRB_32BIT` and `MRB_INT64`

Merge pull request #5137 from shuujii/use-uintptr_t-instead-of-unsigned-long-in-include-mruby-boxing_word.h

Use `uintptr_t` instead of `unsigned long` in `include/mruby/boxing_word.h`

Avoid undefined behavior

Simplify `obj_free`

### 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
  ```

`mrb_bool`  on LLP64 environment seems to become false if, for example,
`mrb_value` is a pointer whose lower 32 bits are 0.

Fix paths of gem files to be scanned for presym

Fix that large integer is not GCed with Word-boxing

When building from outside `MRUBY_ROOT` (e.g. `rake -f`), gem files were not
scanned.