The cloneWC, cloneWanted, cloneImplication family are used by
  * TcHoleErrors
  * TcRule
to clone the /bindings/ in a constraint, so that solving the
constraint will not add bindings to the program. The idea is only
to affect unifications.

But I had it wrong -- I failed to clone the EvBindsVar of an
implication.  That gave an assert failure, I think, as well as
useless dead code.

The fix is easy.  I'm not adding a test case.

In the type 'TcEvidence.EvBindsVar', I also renamed the
'NoEvBindsVar' constructor to 'CoEvBindsVar'.  It's not that we
have /no/ evidence bindings, just that we can only have coercion
bindings, done via HoleDest.