Typechecker for pattern synonyms
Zonk patsyn declarations
Desugaring for pattern synonym declarations
For declarations, the only thing we need to do is desugar the wrapper (if the pattern synonym is bidirectional).
Add pattern synonyms to ModGuts
IfacePatSyn
add bidirectional pattern synonym 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