Add group_recursive field to TyClGroup
ConLike is a type for constructor-like things: real data constructors and pattern synomys.
AConLike is the corresponding new TyThing.
Monoid interface for UniqSet
Typechecker for pattern synonyms
Add pattern synonyms to ModGuts
Add pattern synonym matchers and wrappers to keep_alive list
Handle ConLikes uniformly until we get to mkCoAlgCaseMatchResult
Desugarer for pattern synonym occurances

Split 'CaseAlt ConLike's into DataCon and PatSyn matchers, generate matchers per group
Instantiate PatSyn matchers
IfacePatSyn: Iface representation of an exported pattern synonym