Remove knot-tying bug in TcHsSyn.zonkTyVarOcc
There was a subtle knot-tying bug in TcHsSyn.zonkTyVarOcc, revealed in Trac #15552. I fixed it by * Eliminating the short-circuiting optimisation in zonkTyVarOcc, instead adding a finite map to get sharing of zonked unification variables. See Note [Sharing when zonking to Type] in TcHsSyn * On the way I /added/ the short-circuiting optimisation to TcMType.zonkTcTyVar, which has no such problem. This turned out (based on non-systematic measurements) to be a modest win. See Note [Sharing in zonking] in TcMType On the way I renamed some of the functions in TcHsSyn: * Ones ending in "X" (like zonkTcTypeToTypeX) take a ZonkEnv * Ones that do not end in "x" (like zonkTcTypeToType), don't. Instead they whiz up an empty ZonkEnv.
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