Fix Trac #2856: make deriving work for type families

Fix Trac #2856: make deriving work for type families

Darn, but TcDeriv is complicated, when type families get in on
the act!  This patch makes GeneralisedNewtypeDeriving work
properly for type families.  I think.

In order to do so, I found that GeneralisedNewtypeDeriving can
work for recursive newtypes too -- and since families are conservatively
marked recursive, that's a crucial part of the fix, and useful too.
See Note [Recursive newtypes] in TcDeriv.