Use `TRUE` instead of `1` in mirb code.
Merge pull request #2433 from take-cheeze/mirb_bool_val

Use `TRUE` instead of `1` in mirb code.
Fix MRB_WORD_BOXING by correctly telling GC which mrb_value types are immediate.
Merge pull request #2434 from AE9RB/master

Fix MRB_WORD_BOXING by correctly telling GC which mrb_value types are im...
time overflow check; ref #2337