Use TyVars in PatSyns
I found that some TcTyVars were lurking in a PatSyn, because tc_patsyn_finish was using the TcType -> TcType zonker rather than the TcType -> Type zonker. Eeek. I fixing this I also tided up function naming a bit (still not terrific), and removed the unused TcTyBinder type entirely.
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