Add oneShot demo file

(if you remove {-# GHC_OPTIONS -fno-call-arity #-} then both functions
have the same Core).

Obviously, this patch is not meant to be merged.
Make travis happy
Use oneShot in the definition of foldl etc.
Hack to prevent oneShot from being inlined when simplifying unfoldings
Avoid inlining oneShot in unfoldings
Inline foldl'

(otherwise the oneShot gets lost in the unfolding in the interface)