Implement `-Wcompat`

Currently we have only the following generic warning "levels":

   -W     enable normal warnings
   -w     disable all warnings
   -Wall  enable almost all warnings (details in )

It would be useful to be able to refer collectively to all currently implemented forward-compatibility warnings (e.g. about upcoming changes in base, like -fwarn-amp) via a symbolic -Wcompat flag.

This way, users have more flexibility in opt-in or out of such warnings. So e.g. the following settings are possible in .cabal files:

ghc-options: -Wall :: enable "almost all warnings" as before. Some compatibility warnings may be part of -Wall, some won't (yet). ghc-options: -Wall -Wcompat :: enable "almost all warnings" as well as all currently implemented warnings. E.g. if GHC 8.0 implements -fwarn-mfp-compat but doesn't include it in -Wall, this gives users a way to opt in to get notified early about upcoming changes. ghc-options: -Wall -Wno-compat :: This enables -Wall **minus** any future-compat warnings. This is for users who want to be warned at all about any future compat warnings. ghc-options: -Wcompat :: Of course, -W(no-)compat can be used also w/o -Wall, and then is equivalent to turning on/off the respective individual -fwarn-*-compat flags. ghc-options: -Wall -Wcompat -Wno-mfp-compat :: This would allow to acknowledge the MFP compat warnings, while enabling other future GHC compat warnings.