Refactoring Comparable
Merge pull request #2128 from mirichi/refactoring-comparable

Refactoring Comparable
mruby-string-utf8: use mrb_int instead of size_t
revert 3644f1b temporary since it caused SEGV only on Travis-CI
Refactor flo_finite_p()
Refactor fix_and()
Refactor fix_or()
Refactor fix_xor()