PERF: speed up factorization of categoricals
Factorizing Categoricals/CategoricalIndexes in ``_factorize_from_iterable`` is unneccesary slow, because a unneeded CategoricalIndex is created. This fixes that.

```python
>>> n = 1_000_000
>>> c = pd.Categorical(list('a' * n) + (['bcd'] * n), categories=['a', 'bcd'])
>>> %timeit pd.core.arrays.categorical._factorize_from_iterable(c)
120 µs  # master
2.84 µs  # this PR
```
