Fix and document cloneWC
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.
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