Implemented levenshtein

I had to do some awful hack to get the same (stupid) parsing semantics as PHP (see w_const_inst).

Tests run:
```bash
$ PYTHONPATH=~/test_hippy/pypy py.test -r s test_phpt/ext/standard/strings/levenshtein.phpt test_phpt/ext/standard/general_functions/003.phpt
```