[PROPOSAL] Merge `deepseq-generics` into `deepseq`

**Formal proposal to `libraries@`: http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/23031**

This also replaces the existing `rnf x = seq x ()` default
implementation with a `Generics`-based `DefaultSignature` `rnf` method
implementation

This requires to drop support for GHCs older than GHC 7.2 to avoid
conditional exports due to lack of `Generics` support
Is the goal to stay API compatible by deepseq-generics so users can depend on

```
if flag(deepseq-generics)
  build-depends:
    deepseq < 1.4
    deepseq-generics >= Y && < Z
else:
  build-depends:
    deepseq >= 1.4
```

and always import `Control.DeepSeq.Generics`? If so add some documentation (just like we did for network when `Network.URI` was split off) telling users how to specify the dependencies correctly in their .cabal files.