Explicitly sign- or zero-extend the result of a ccall up to the word
size if necessary.  Recent discussion on
glasgow-haskell-users@haskell.org suggests that this is the
responsibility of the caller rather than the callee.

We do it by wrapping the result in narrow{8,16,32}{Int,Word}# as
appropriate, at desugaring time, because this way we only have to do
it once instead of once per backend.  Furthermore the narrowing is
exposed to the simplifier which is generally a good thing.