Renamer for pattern synonym declarations

The right-hand side creates the pattern scope in which the left-hand
side is processed
Internal representation of PatSyn data
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
IfacePatSyn
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
Fully store pattern synonym declarations in interface file (WIP)