Refactor and document conditional type binder

[ilevkivskyi]

Conditional type binder (mypy's version of abstract interpreter) is one of the trickiest part of mypy. It would be great to clean-up its implementation and document the current behaviour. As and optional additional related thing (or a follow-up step), I would propose to refactor/rethink partial types (that IMO should be handled by the binder).