Rewrite String#tr_trans to improve the 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(#3015) as well