In tcFamTyPats we were zonking from the TcType world to the
Type world, ready to build the results into a CoAxiom (which
should have no TcType stuff.  But the 'thing_inside' for
tcFamTyPats also must be zonked, and that zonking must have
the ZonkEnv from the binders zonked tcFamTyPats.

Ugh.  This caused an assertion failure (with DEBUG on) in
RaeBlobPost and TypeLevelVec, both in tests/dependent, as
shown in Trac #12682.  Why it hasn't shown up before now
is obscure to me.

So I moved the zonking stuff out of tcFamTyPats to its
three call sites, where we can do it all together. Very
slightly longer, but much more robust.