Rewrite tr_trans mehold to improve performance

    - I had add a new private method tr_trans_ascii_only to deal
      with ascii only string.
    - In order to get the best performance, I did not follow the DRY
      rule.
    - I also did some benchmark. you can check them.
      (https://gist.github.com/felix125/11235967)
    - Solve multiple char issue also.
      (https://github.com/rubinius/rubinius/issues/3015)
    - Add some specs, and i think we need more specs!!