Fix Trac #3087: derived Data now defines dataCast1/2

Fix Trac #3087: derived Data now defines dataCast1/2

This patch generates code in deriving(Data) for dataCast1 or 2 as
appropriate.

While I was there I did some refactoring (of course), pulling out
the TcDeriv.inferConstraints as a separate function.

I don't think it's worth merging this to 6.10.2, even though it's a bugfix,
because it modifies code that I added in the HEAD only (for deriving Functor)
so the merge will be sligtly awkward.  And there's an easy workaround.