Revised FFI syntax + typing

[Simon Marlow]

> > Why is Word a GHC extension?  Someone remind me?
> 
> Hmmm, I thought that the Word type itself is a GHC extension, the
> hslibs docs for module Word only talk about the explictly sized
> variants. OTOH the implementation exports Word, but for legacy
> reasons, IIRC. If this is really the case, we should document this
> somehow in the sources.

I don't think I have any strong feelings either way.  Word has a
pleasing symmetry with Int, but on the other hand if it isn't useful
then we should omit it.  I agree in principle with Alastair's
portability argument, but don't think it is worth applying for this
particular case since we already have Int.

Cheers,
	Simon