[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
I'm curious why this is needed. In the generic hashable instances you just have to write:

```haskell
 {-# LANGUAGE DeriveGeneric #-}

 import GHC.Generics (Generic)
 import Data.Hashable

 data Foo a = Foo a String
              deriving (Eq, Generic)

 instance Hashable a => Hashable (Foo a)
```

Note the lack of a body in the `Hashable` instance declaration.