More WFA cleanups.

Add some much smarter ways to compute df_in_dataset and
tf_in_dataset, and clean up the option parsing.
Some more small cleanups to WFA.
Fix class names: Segmenter -> Segments.

This was a silly inconsistency, change it.
Refactor WFA into the domain layer.

Mostly just a move, though we've also now got its spec working
as a fast-spec.