Add missing kind-check for tcEqType on forall-types

This wasn't showing up as a bug, but it was definitely wrong.
Don't export isTcReflCo_maybe (unused)
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For equalities with incompatible kinds, new IrredCan goes in the inert set, not work list

This change makes the code for canIrred markedly simpler (and more efficient)
See Note [Equalities with incompatible kinds].

I don't think there was really a bug here, but I came across it when
fixing Trac #8913
Debug tracing only
Flattener preserves synonyms, rewriteEvidence can drop buggy "optimisation"

There was a special case in rewriteEvidence, looking like:
  = return (Just (if ctEvPred old_ev `tcEqType` new_pred
                  then old_ev
                  else old_ev { ctev_pred = new_pred }))

But this was wrong: old_pred and new_pred might differ in the kind
of a TyVar occurrence, in which case tcEqType would not notice,
but we really, really want new_pred.  This caused Trac #8913.

I solved this by dropping the whole test, and instead making
the flattener preserve type synonyms. This was easy because
TcEvidence has TcTyConAppCo which (unlike) Coercion, handles
synonyms.
Implicit parameters should not be allowed in class and instance declarations

Trac #8912 pointed out that GHC 7.4 and 7.6 have omitted this test, although
7.2 and earlier had it.  This patch puts the test back in, and refactors a
little.
Comments only