Reform the free variable finders for types

This patch delivers on (much of) #17509.

* Introduces the shallow vs deep free variable distinction

* Introduce TyCoRep.foldType,
    foldType :: Monoid a => TyCoFolder env a
                         -> env -> Type -> a
  and use it in the free variable finders.

* Substitution in TyCoSubst
   * ASSERTs are on for checkValidSubst
   * checkValidSubst uses shallowTyCoVarsOfTypes etc

Quite a few things still to do

* We could use foldType in lots of other places

* We could use mapType for substitution.  (Check that we get
  good code!)

* Some (but not yet all) clients of substitution can now
  save time by using shallowTyCoVarsOfTypes

* All calls to tyCoVarsOfTypes should be inspected; most of
  them should be shallow.  Maybe.

* Currently shallowTyCoVarsOfTypes still returns
  unification variables, but not CoVarHoles.
  Reason: we need to return unification variables
  in some of the calls in TcSimplify, eg when promoting.

* We should do the same thing for tyCoFVsOfTypes, which is
  currently unchanged.

* tyCoFVsOfTypes returns CoVarHoles, because of the
  use in TcSimplify.mkResidualConstraints.  See
  Note [Emitting the residual implication in simplifyInfer]

* #17509 talks about "relevant" variables too.

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