Add method `toolchains` and `primary_toolchain` to `MRuby::Build`.
* To check imported toolchains for setting up toolchain specific things.
* `primary_toolchain` returns the main toolchain name of the build.
Use `MRB_ARGS_ARG(n, o)` instead of `MRB_ARGS_REQ(n) | MRB_ARGS_OPT(o)`.
Merge pull request #2436 from take-cheeze/mrb_args_arg

Use `MRB_ARGS_ARG(n, o)` instead of `MRB_ARGS_REQ(n) | MRB_ARGS_OPT(o)`.
Ensure RVALUE is large enough for word boxing types.
Merge pull request #2438 from AE9RB/upstream

Ensure RVALUE is large enough for word boxing types.
fiber stack should not be cleared by zero when MRB_NAN_BOXING is set; close #2439
Merge branch 'master' of github.com:mruby/mruby
Add a caution to Your Pull Request document.
Merge pull request #2440 from suzukaze/add-pr-docs2

Add a caution to Your Pull Request document. (suggested by Matz) close #2437
Use RSTR_NOFREE_P in state.c
Use *_EMBED_* macro in state.c
Merge pull request #2432 from take-cheeze/toolchains

Add method `toolchains` and `primary_toolchain` to `MRuby::Build`.
Merge pull request #2443 from suzukaze/use-mrb-str-emebed-macro

Use *_EMBED_* macro in state.c
Merge pull request #2442 from suzukaze/use-mrb-str-nofree-macro

Use RSTR_NOFREE_P in state.c