Tidy up and refactor wildcard handling
When examining #10615, I found the wildcard handling hard to understand. This patch refactors quite a bit, but with no real change in behaviour. * Split out TcIdSigInfo from TcSigInfo, as a separate type, like TcPatSynInfo. * Make TcIdSigInfo express more invariants by pushing the wildard info into TcIdSigBndr * Remove all special treatment of unification variables that arise from wildcards; so the TauTv of TcType.MetaInfo loses its Bool argument. A ton of konck on changes. The result is significantly simpler, I think.