Make findPrototypeId faster

[tuchida]

ref. #837 (comment)

It seems to be faster to use Java7's strings switch for findPrototypeId.
master...tuchida:improve-findPrototypeId
https://gist.github.com/tuchida/a8141c9946b0b597cdb2b171a3b8be5f

I think it is better to change to strings switch as it is more maintainable than generating code with org.mozilla.javascript.tools.idswitch.Main.
If there seems to be no problem, I will create a Pull Request.

[rbri]

Have done a short test by hacking SwitchGenerator to generate your switch. After regenerating nearly all classes i found no real difference when running Test262SuiteTest.

Will do some more testing... but if this seems to have no negative effect maybe we can use this to have a more cleaner code.

[tuchida]

Thank you for fixed and merged.