Add PatSynDecl to TyClDecl
Parser: add pattern_synonym_decl
Renamer for pattern synonym declarations

The right-hand side creates the pattern scope in which the left-hand
side is processed
PatSynDecls don't introduce names to the TyCl namespace
Internal representation of PatSyn data
Typechecker for pattern synonyms, called from tcTyClsInstDecls
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
Parser: pattern exports
Add pattern synonyms to md_decls
IfacePatSyn
Globalise wrapper ID
add bidirectional pattern synonym wrappers to keep_alive list