Refactor mrb_str_new() in string.c
Refactor mrb_str_equal_m in string.c
Refactor mrb_str_intern() in string.c
Merge pull request #2275 from suzukaze/refactor-string.c-2

Refactor string.c Fix #2267
Add comment to Fixnum Class.
Merge pull request #2276 from yui-knk/add-comment

Add comment to Fixnum Class.