Improve pre-empty-case situation

[treeowl]

• Move all the empty case CPP crud into its own module. We just
  need to define a single absurd1 :: V1 a -> b function.

• Use pseq to guarantee that the compat code will actually
  produce the correct bottom.

[RyanGlScott]

Thanks! There's one build failure on GHC 7.0:

[ 5 of 23] Compiling Generics.Deriving.Absurd ( src/Generics/Deriving/Absurd.hs, /__w/generic-deriving/generic-deriving/dist-newstyle/build/x86_64-linux/ghc-7.0.4/generic-deriving-1.14.1/build/Generics/Deriving/Absurd.o )

src/Generics/Deriving/Absurd.hs:11:12:
    Not in scope: type constructor or class `V1'

This can likely be fixed by importing from Generics.Deriving.Base instead of GHC.Generics.