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

**NB: Formal proposal to `libraries@` coming soon**

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