Move foldlStrict (defined 4 times) to Data.StrictFold.

The foldlStrict is Data.List.foldl' which is always inlined, which
allows more optimizations. Also, foldl' is not Haskell 98, although
it is Haskell 2010.
Move utilities (BitUtils,Strict{Fold,Pair}) to Utils directory.

The Data directory was becoming a little too crowded.