Implemented str_shuffle

The implementation is ready, however the shuffling code needs to be refactores as it the same as in array/shuffle. I tried to extract the method in array and import the array/funcs module, but it is not possible due to circular imports (array also imports string). What could be a good place to put that extracted method? Is there a utils module where such stuff can be put?

Tests run:
```bash
 PYTHONPATH=~/test_hippy/pypy py.test -r s test_phpt/ext/standard/strings/str_shuffle* test_phpt/ext/standard/strings/004.phpt
```