make forcats.py its own library

[machow]

This module implements convenience functions for working with pandas factors. As such, all of its behaviors are useful outside of siuba. Its responsibilities look like...

outside siuba: convert Series/Cats (or arraylikes) -> Cats

inside siuba: return a symbolic call when it's first argument is a Symbol (e.g. _)

A big win here is that this will illustrate how siuba can use external tools / libraries.