Improve TcFlatten.flattenTyVar

This patch tides up the structure, simplifying FlattenTvResult.

It also replaces a use of zonkTcType (which I hated) with
coercionKind, in that same function.  Happily, the result is
little faster, maybe even a percentage point or two, which is
a lot for a compiler.

This also removes the line
   || not (map binderVisibility bndrs1 == map binderVisibility bndrs2)
from TcCanonical.can_eq_nc', in the ForAllTy/ForAllTy case.

Why? Becuase I can't see why binder-visiblity should matter, and
when we use coercionKind instead of zonkTcType in flattenTyVar,
this case pops up and rejects a program that should pass.  I did
discuss this with Richard.

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