[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
Don't we already have a precendent for not allowing auto-generatde instances for empty data-classes? For instance,

```hs
data Void deriving Show
```

fails with

```
    Can't make a derived instance of ‘Show Void’:
      ‘Void’ must have at least one data constructor
      Possible fix: use a standalone deriving declaration instead
    In the data declaration for ‘Void’
```

Same error when trying to derive the other built-in classes (`Eq`, `Ord`, `Enum`, `Bounded`, `Read`)

Moreover, if we add that instance now without a real demand, we'll have a much harder time removing it lateron (major version bump + code breakage for removing an instance vs. minor bump for merely adding an instance). But we can always add it lateron cheaply, if it can be shown we really need/want that one.