Remove useless comments.
Clean up variable types and type casts.
Remove a comment.
Use memcmp instead of strcmp if it is possible.
Define mrb_code, mrb_aspec as uint32_t. They are always positive.
Use mrb_aspec instead of int32_t. As it will clarify what is doing.
Merge pull request #1195 from monaka/pr-cleanup-string.c-20130421

Cleanup string.c 20130421
Merge pull request #1196 from monaka/pr-use-mrb_aspec

Cleanup around mrb_code, mrb_aspec.