Internal representation of PatSyn data
Typechecker for pattern synonyms, called from tcTyClsInstDecls
TyThing: add APatSyn, add ConLike

ConLike is a type for constructor-like things: real data constructors
and pattern synomys.
typecheck & instantiate pattern synonym patterns in patterns and expressions
Desugaring for SynPatOut and PatSynDecl
IdDetails: add PatSynWrapId