Rename STR_BUF_MIN_SIZE to MRB_STR_BUF_MIN_SIZE. Make it configurable.
Remove mrb_str_new2(). Use mrb_str_new_cstr() instead.
 Make mrb_str_new_cstr() accept NULL pointer. It generates 0byte strings by NULL pointer.
Remove mrb_str_sublen() as it is not used.
Remove str_alloc(). Use mrb_obj_alloc_string instead.
Merge pull request #982 from monaka/pr-make-MRB_STR_BUF_MIN_SIZE-configurable

Rename STR_BUF_MIN_SIZE to MRB_STR_BUF_MIN_SIZE. Make it configurable.
Merge pull request #984 from monaka/pr-make-mrb_str_new2-obsolete

Remove mrb_str_new2().
Merge pull request #985 from monaka/pr-remove-unused-code-20130312

Remove mrb_str_sublen() as it is not used.
Merge pull request #986 from monaka/pr-use-mrb_obj_alloc_string-instead-of-str_alloc

Remove str_alloc(). Use mrb_obj_alloc_string() instead.