add MRB_INT_BIT

I initially assumed `sizeof(mrb_int)*CHAR_BIT` is more often used, but that wasn't the case.

But I still think `MRB_INT_BIT` could be an useful addition:
- `<limits.h>` for `CHAR_BIT` is not needed anymore
- easier and faster to read than `sizeof(mrb_int)*CHAR_BIT`
- simplifies preprocessor conditionals: `defined MRB_INT16 || defined MRB_INT64` → `MRB_INT_BIT != 32`