[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 find this sentence a bit hard to parse. What should I do if I 1) want to be backwards compatible and what should I do if 2) I don't want to be backwards compatible.