[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
Because that's an old code example from `deepseq-generics`; I've actually stolen that `Hashable` `Foo` example and placed it directly into the `NFData` haddock string (replacing `Hashable` by `NFData`)

I'll need to add some notes to the example you quoted noting you don't need to explicitly `where rnf = genericRnf` anymore, unless you need backward-compatibility with an older `deepseq`+`deepseq-generics`-combo