Orient TyVar/TyVar equalities with deepest on the left
Trac #15009 showed that, for Given TyVar/TyVar equalities, we really want to orient them with the deepest-bound skolem on the left. As it happens, we also want to do the same for Wanteds, but for a different reason (more likely to be touchable). Either way, deepest wins: see TcUnify Note [Deeper level on the left]. This observation led me to some significant changes: * A SkolemTv already had a TcLevel, but the level wasn't really being used. Now it is! * I updated added invariant (SkolInf) to TcType Note [TcLevel and untouchable type variables], documenting that the level number of all the ic_skols should be the same as the ic_tclvl of the implication * FlatSkolTvs and FlatMetaTvs previously had a dummy level-number of zero, which messed the scheme up. Now they get a level number the same way as all other TcTyVars, instead of being a special case. * To make sure that FlatSkolTvs and FlatMetaTvs are untouchable (which was previously done via their magic zero level) isTouchableMetaTyVar just tests for those two cases. * TcUnify.swapOverTyVars is the crucial orientation function; see the new Note [TyVar/TyVar orientation]. I completely rewrote this function, and it's now much much easier to understand. I ended up doing some related refactoring, of course * I noticed that tcImplicitTKBndrsX and tcExplicitTKBndrsX were doing a lot of useless work in the case where there are no skolems; I added a fast-patch * Elminate the un-used tcExplicitTKBndrsSig; and thereby get rid of the higher-order parameter to tcExpliciTKBndrsX. * Replace TcHsType.emitTvImplication with TcUnify.checkTvConstraints, by analogy with TcUnify.checkConstraints. * Inline TcUnify.buildImplication into its only call-site in TcUnify.checkConstraints * TcS.buildImplication becomes TcS.CheckConstraintsTcS, with a simpler API * Now that we have NoEvBindsVar we have no need of termEvidenceAllowed; nuke the latter, adding Note [No evidence bindings] to TcEvidence.
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