Define type mrb_bool. It is typedef-ed to _Bool on C99, unsigned int on MSVC.
It is safer than applying 1bit bit-fields to signed int.

For forward compatibility, you should substiture only 1 or 0 for the variable typed mrb_bool.
Merge pull request #997 from monaka/pr-add-type-mrb_bool_t

Define mrb_bool_t.