Use the eager unifier in the constraint solver

This patch continues the refactoring of the constraint solver
described in #23070.

The Big Deal in this patch is to call the regular, eager unifier from the
constraint solver, when we want to create new equalities. This
replaces the existing, unifyWanted which amounted to
yet-another-unifier, so it reduces duplication of a rather subtle
piece of technology. See

  * Note [The eager unifier] in GHC.Tc.Utils.Unify
  * GHC.Tc.Solver.Monad.wrapUnifierTcS

I did lots of other refactoring along the way

* I simplified the treatment of right hand sides that contain CoercionHoles.
  Now, a constraint that contains a hetero-kind CoercionHole is non-canonical,
  and cannot be used for rewriting or unification alike.  This required me
  to add the ch_hertero_kind flag to CoercionHole, with consequent knock-on
  effects. See wrinkle (2) of `Note [Equalities with incompatible kinds]` in
  GHC.Tc.Solver.Equality.

* I refactored the StopOrContinue type to add StartAgain, so that after a
  fundep improvement (for example) we can simply start the pipeline again.

* I got rid of the unpleasant (and inefficient) rewriterSetFromType/Co functions.
  With Richard I concluded that they are never needed.

* I discovered Wrinkle (W1) in Note [Wanteds rewrite Wanteds] in
  GHC.Tc.Types.Constraint, and therefore now prioritise non-rewritten equalities.

Quite a few error messages change, I think always for the better.

Compiler runtime stays about the same, with one outlier: a 17% improvement in T17836

Metric Decrease:
    T17836
    T18223

1 files changed, 4 insertions, 4 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T18851c.stderr b/testsuite/tests/typecheck/should_fail/T18851c.stderr
index 58c15d1b77..ebe5f8621d 100644
--- a/testsuite/tests/typecheck/should_fail/T18851c.stderr
+++ b/testsuite/tests/typecheck/should_fail/T18851c.stderr
@@ -2,13 +2,13 @@
 T18851c.hs:25:27: error: [GHC-25897]
     • Could not deduce ‘n2 ~ n1’
         arising from reasoning about an injective type family using constraints:
-          ‘Plus1 n2 ~ n’
-            arising from a type equality
-              VSucc (Plus1 n2) ~ VSucc n at T18851c.hs:25:27-33
           ‘Plus1 n1 ~ n’
+            arising from a type equality
+              VSucc (Plus1 n1) ~ VSucc n at T18851c.hs:25:27-33
+          ‘Plus1 n2 ~ n’
             arising from a pattern with constructor:
                            VSucc :: forall (n :: Nat). V n -> VSucc (Plus1 n),
-                         in an equation for ‘foo’ at T18851c.hs:25:6-12
+                         in an equation for ‘foo’ at T18851c.hs:25:16-22
       from the context: n ~ Plus1 n1
         bound by a pattern with constructor:
                    VSucc :: forall (n :: Nat). V n -> VSucc (Plus1 n),