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
Reject pattern synonym declarations in hs-boot files, add them to exportable names
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