Refactor StringMethods for extension arrays

[TomAugspurger]

This is an issue to track a refactor of StringMethods that'll be necessary for ArrowStringArray to use pyarrow compute algorithms.

We'll need to update StringMethods

1. Extract the array from the Series / Index
2. Dispatch the string methods
3. Wrap the result

So in my proposal, we'll have something like

class StringMethods:
    def __init__(self, data, ...):
        self._array = data.array
        ...

    def upper(self):
        return self._wrap_result(self._array._str.upper())

cc @xhochy. I have a branch started and will hopefully finish it off this week.