Added several strings Optimizers.

Hi, 

I have added several strings relative optimizers , using internal function not PHP userland function call.

List: `ExplodeOptimizer` , `ImplodeOptimizer`, `LtrimOptimizer`, `RtrimOptimizer`, `StrposOptimizer`, `FileGetContentsOptimizer` ..

And all optimizers compatible PHP userland function parameters. 

Use case, please ref `test/strings.zep` and `unit-tests/strings.php`