Fix bug in the short-cut solver

Trac #13943 showed that the relatively-new short-cut solver
for class constraints (aka -fsolve-constant-dicts) was wrong.
In particular, see "Type families" under Note [Shortcut solving]
in TcInteract.

The short-cut solver recursively solves sub-goals, but it doesn't
flatten type-family applications, and as a result it erroneously
thought that C (F a) cannot possibly match (C 0), which is
simply untrue.  That led to an inifinte loop in the short-cut
solver.

The significant change is the one line

+                 , all isTyFamFree preds  -- See "Type families" in
+                                          -- Note [Shortcut solving]

but, as ever, I do some other refactoring.  (E.g. I changed the
name of the function to shortCutSolver rather than the more
generic trySolveFromInstance.)

I also made the short-cut solver respect the solver-depth limit,
so that if this happens again it won't just produce an infinite
loop.

A bit of other refactoring, notably moving isTyFamFree
from TcValidity to TcType