rename `mrb_str_buf_append` to `mrb_str_cat_str`

The new name is better and less confusing, because:
  - `mrb_str_append` calls `mrb_str_to_str` and this function doesn't
  - `mrb_str_buf_append` _is_ `mrb_str_cat` for `mrb_value` strings
Merge pull request #2142 from cremno/rename-mrb_str_buf_append

rename `mrb_str_buf_append` to `mrb_str_cat_str`