Fix two obscure bugs in rule matching

This patch fixes Trac #14777, a compiler crash.

There were actually two bugs.

1. In Rules.matchN, I was (consciously) not rename the template binders
   of the rule. Sadly, in rare cases an accidental coincidence of
   uniques could mean that a term variable was mapped to a type
   variable, utterly bogusly.  See "Historical note" in
   Note [Cloning the template binders] in Rules.

   This was hard to find, but easy to fix.

2. The fix to (1) showed up a bug in Unify.hs.  The test in
   Unify.tvBindFlag was previously using the domain of the RnEnv2
   to detect locally-bound variables (e.g. when unifying under
   a forall).  That's fine when teh RnEnv2 starts empty, as it
   does in most entry points.  But the tcMatchTyKisX entry point,
   used from the rule matcher, passes in a non-empty RnEnv2 (by
   design).  Now the domain of the RnEnv doesn't idenfity those
   locally-bound variables any more :-(.

   Solution: extend UmEnv with a new field um_skols, to capture
   the skolems directly.  Simple, easy, works.

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