### 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

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