new language extensions

[Duncan Coutts]

All,

GHC 6.8 has a lot of new names for language extensions which are not yet
listed in Language.Haskell.Extension:

UnicodeSyntax
PatternSignatures
UnliftedFFITypes
LiberalTypeSynonyms
TypeOperators
PArr
RecordWildCards
RecordPuns
DisambiguateRecordFields
OverloadedStrings
GADTs
MonoPatBinds
RelaxedPolyRec
ExtendedDefaultRules
UnboxedTuples
DeriveDataTypeable
ConstrainedClassMethods

These are the extension names you can list in the "extensions:" field in
a .cabal file and are also in the {-# LANGUAGE #-} pragma.

So I propose we add all of these to the Extension enumeration data type.

I'd like to keep this addition separate from the proposal to make
Extension a String or newtype String, since I expect even in that case
we need a central list of extensions so different implementations can
agree on names when they implement the same extensions, or to ensure
they pick different names for different extensions.

It'd also be nice if we could document all these extensions in the
Language.Haskell.Extension module so we can refer to it from the cabal
documentation about the extensions field.

Duncan