Refactoring.

* Use oniguruma API directly since oniguruma posix API breaks the stack sometime.
* Use *MRB_SET_INSTANCE_TT* to store C struct directly to object.
* Add *OnigRegexp.version* to get oniguruma's version.
* Implement *OnigMatchData* class natively.
* Add tests for *OnigMatchData*.
* Replace *mrb_intern_cstr* with *mrb_intern_lit*.