Implemented by adding `OP_HASHCAT` that merges hashes.

`gcc -O3` raises error on truncation using `snprintf`.

This time we used `Integral` module which is mruby specific.

`"a".sub!("a", "a")` should not return `nil`.

`Hash#compact!` should return `nil` if the receiver does not change.

`mrb_iv_p` -> `mrb_iv_name_sym_p`
`mrb_iv_check` -> `mrb_iv_name_sym_check`

Use segment list for instance variable again to reduce memory.

The macro `RCLASS_SUPER`, `RCLASS_IV_TBL` and `RCLASS_M_TBL` are
removed from `include/mruby/class.h`.

Unless `MRB_DISABLE_STDIO` is set. `snprintf` is used anyway
if mruby is configured to use `stdio`. This change reduces 8KB
of program size on the Linux box.

This means reducing one word per a call frame.

Renamed flag macro names as well:
`MRB_FLAG_IS_FROZEN` -> `MRB_FL_OBJ_FROZEN`
`MRB_FLAG_IS_PREPENDED` -> `MRB_FL_CLASS_IS_PREPENDED`
`MRB_FLAG_IS_ORIGIN` -> `MRB_FL_CLASS_IS_ORIGIN`
`MRB_FLAG_IS_INHERITED` -> `MRB_FL_CLASS_IS_INHERITED`

Although the return value is not defined in ISO, it is better to behave
as CRuby does.

`gcc -O3` raises error on truncation using `snprintf`.

This time we used `Integral` module which is mruby specific.

Fix misspelling words in comments, docs

`"a".sub!("a", "a")` should not return `nil`.