Split elements in the input stream into two parts using a pure
splitter function, direct each part to a different fold and zip the
results.
Definitions:
>>> unzipWith f = Fold.unzipWithM (return . f)
>>> unzipWith f fld1 fld2 = Fold.lmap f (Fold.unzip fld1 fld2)
This fold terminates when both the input folds terminate.
Pre-release