Track rewriter sets more accurately in constraint solving

The key change, which fixed #25440, is to call `recordRewriter` in
GHC.Tc.Solver.Rewrite.rewrite_exact_fam_app.  This missing call meant
that we were secretly rewriting a Wanted with a Wanted, but not really
noticing; and that led to a very bad error message, as you can see
in the ticket.

But of course that led me into rabbit hole of other refactoring around
the RewriteSet code:

* Improve Notes [Wanteds rewrite Wanteds]

* Zonk the RewriterSet in `zonkCtEvidence` rather than only in GHC.Tc.Errors.
  This is tidier anyway (e.g. de-clutters debug output), and helps with the
  next point.

* In GHC.Tc.Solver.Equality.inertsCanDischarge, don't replace a constraint
  with no rewriters with an equal constraint that has many. See
  See (CE4) in Note [Combining equalities]

* Move zonkRewriterSet and friends from GHC.Tc.Zonk.Type into
  GHC.Tc.Zonk.TcType, where they properly belong.

A handful of tests get better error messages.

For some reason T24984 gets 12% less compiler allocation -- good

Metric Decrease:
    T24984