Adds some extra filters and a way to combine filters.

Also adds documentation and testing.
Tricky. It's identical in structure to monadic bind, but it has a restricted type signature. I don't know whether it's best to change the name to, say, `andThen`, to reflect the similarities, or to _avoid_ doing so, to reflect the differences.

But regardless, at the very least I'd expand the comment a bit to be more specific.