Use explicit language extensions in base

[Bas van Dijk]

Hello,

I just wrote a patch that removes the extension fields from base.cabal
and adds LANGUAGE pragmas where needed:

http://hackage.haskell.org/trac/ghc/ticket/4925

This has the advantage that modules become more modular (since they
don't need to depend on external make or cabal files which specify how
to build them).

For the record: this is not a library proposal. It's just an internal
patch for base. It would be great if someone could review and apply
it.

Regards,

Bas