Define mrb_bool_t.

It's for compatibility improvements.

It's hard to predict what value is stored on "signed int" members with 1bit bit-field. Their behaviors are completely runtime dependent.
And there are some unstable code like this in the mruby core.

I suggest to use new type mrb_boot_t.
It is typedef-ed to _Bool on C99. _Bool is well defined in C99 spec.
So low risks. And it except some optimization by compilers.

Meanwhile, as always, there is no _Bool on MSVC (sigh). So it is typedef-ed to "unsigned int".
You might think it is small fix. But it is safer than current code. And it may affords intensions of the author.