Modern scoped type variables: extension shuffling

The accepted GHC Proposal #448 "Modern scoped type variables" contains a section on "Extension shuffling" that redistributes responsibility of ScopedTypeVariables extension between new ones, introduces new extension and warning, in particular:

• PatternSignatures allows user to write signatures in patterns
• MethodTypeVariables gives variables from the instance head into method scope
• ExtendedForAllScope gives any forall'ed type variables into expression scope
• ImplicitForAll allows user to turn on/off implicit forall creation
• -Wpattern-signature-binds warns if pattern signature binds a type variable