[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
Now that I think of it, having `Control.DeepSeq.Generics`  in `deepseq` is in fact optional.

While it may be useful to have a helper `genericRnf` if you want to `rnf` something on-the-fly w/o having to define an instance first (e.g. to avoid orphans), `Control.DeepSeq.Generics` can very well continue to live on in `deepseq-generics` for those who need it, as it doesn't need to share the *same* `Control.DeepSeq.Generics`. And we'd avoid to handle those awkward moving-module-between-package handling on the user side.