PERF: categorical rank
xref https://github.com/pandas-dev/pandas/pull/15422#discussion_r102941126

easy enough after #15422 to rank the categories themselves rather than using expanded values; prob most relevant for ``object`` dtypes.

```
In [15]: s = Series(tm.makeCategoricalIndex(100000))

In [16]: res = Series(np.array(s.cat.rename_categories(Series(s.cat.categories).rank()))).rank()

In [17]: res2 = s.rank()

In [18]: res.equals(res2)
Out[18]: True

In [19]: %timeit Series(np.array(s.cat.rename_categories(Series(s.cat.categories).rank()))).rank()
100 loops, best of 3: 4.39 ms per loop

In [20]: %timeit s.rank()
10 loops, best of 3: 132 ms per loop
```