invalid base shouldn't silently fail

remove unused `mrb_str_body`

Revert 44a58092 and check range of
`base` with `mrb_assert`.
It actually can only be either 2, 8, 10 or 16 (and never 0), unless
mruby sources are modified. But I don't see a reason to impose further
limits.

Use `mrb_print_error` in test error handling.

Remove unnecessary out of memory check in `ary_expand_capa`.

Add new API `mrb_ary_resize`(mruby implementation of `rb_ary_resize`).

Since `mrb_realloc` raises when failed to allocate memory.

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)`.