universially-bound tyvars are in scope when renaming existentially-bound
tyvars in a pattern synonym signature
tcTySig for PatSynSigs: filter out universially-bound type variables from ex_tvs
Show foralls (when requested) in pattern synonym types
Add TcPatSynInfo as a separate type (same pattern as PatSynBind being a separate type)
Create SigTyVars for existentials under tcPat when typechecking a PatSyn
with a type signature