Support parsing a Regexp literal with 'o' option

Refactor `mrb_type` in `include/mruby/boxing_word.h`

`byteslice` creates 2 string objects. `_bufread` creates one, and
modifies the original buffer string, that is more efficient.

Use `%C` to `mrb_name_error` in `mrb_method_search`; ref 5890c7d5

The `bison` on MacOS does not support `%define api.pure`.
This reverts commit f7c9f1f7.

In the typical case, `mrb_funcall` invocation would be skipped.

Revert part of "Refine `tasks/toolchains/(gcc|clang).rake`" (07c6b7f0)

`-Wzero-length-array` is not a mandatory option, so the original is better.

Fix warnings for invalid C++ option with `enable_cxx_abi`; ref #3618, #4703

To stop warnings since C++ do not accept `-std=gnu99` option.

Refine `tasks/toolchains/(gcc|clang).rake`

- Make sure to specify `-std=gnu99` for C compiler flag.
- Make sure to specify `-Wzero-length-array` for C/C++ compiler flag (Clang).
- Extract similar codes.

1. `$/` and other Perl-ish global variables are not defined in ISO.
2. The current Ruby policy do not encourage those variables.
3. Those variables has global effect and can cause troubles.

Add paragraph mode to String#each_line in mrblib

The `Rakefile` was a leftover from the time it was an independent gem.

E.g. `io.read(5)` should read 5 byte string, not 5 characters.

Unlike CRuby, there's no way to process strings byte-wise by core
methods because there's no per string encoding in mruby, so that
we moved 3 byte-wise operation methods from `mruby-string-ext` gem.

Merge pull request #4701 from shuujii/drop-test-dependency-from-mruby-string-ext-to-mruby-enumerator

Drop test dependency from `mruby-string-ext` to `mruby-enumerator`

Refine documentation for C99 compliance; ref #4685 [ci skip]

The refinement was based on comment from @matz.

add special local variable _ in mirb