Set up the test enviroment

    - The spec is copy from mspec and convert to minitest compatible
      version.
Add tr_s_spec.rb

    - This spec is copy fome mspec and convert to minitest compatible
      version.
Rewrite tr_trans

    - 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 add some benchmark. you can check them.
    - Solve multiple char issue #3015