[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
I'm not sure how to best explain it. It's mostly to statically detect defining `NFData`  for uninhabited types which I consider a code-smell. See also

  https://hackage.haskell.org/package/deepseq-generics-0.1.1.1/docs/Control-DeepSeq-Generics.html#v:genericRnfV1

for an example