Reduce allocation in shared RArray clearing.

Fix type checking in `mrb_flo_to_fixnum`.

Always return non-NULL in `mrb_hash_tbl`.

Use RSTR_NOFREE_P in state.c

Use *_EMBED_* macro in state.c

Add method `toolchains` and `primary_toolchain` to `MRuby::Build`.

Add a caution to Your Pull Request document. (suggested by Matz) close #2437

Ensure RVALUE is large enough for word boxing types.

Use `MRB_ARGS_ARG(n, o)` instead of `MRB_ARGS_REQ(n) | MRB_ARGS_OPT(o)`.

Fix MRB_WORD_BOXING by correctly telling GC which mrb_value types are im...

Use `TRUE` instead of `1` in mirb code.

* To check imported toolchains for setting up toolchain specific things.
* `primary_toolchain` returns the main toolchain name of the build.

Reduce warning for enumeration

22 enumeration values not handled in switch: 'MRB_TT_FALSE',
'MRB_TT_FREE', 'MRB_TT_TRUE'... [-Wswitch]

Sacrifice some micro-optimization to remove duplicate code

* Filter `MRB_TT_ENV` and `MRB_TT_ICLASS`.
* Set `mrb->string_class` in `mrb_init_exception` instead.

Fix typo in doc/api/mruby.h.md

Document rest mrbconfs.