Missing instance declarations in base

[Henning Thielemann]

On Mon, 29 Jun 2015, Edward Kmett wrote:

> It depends. The problem is ultimately that folks use Storable for many 
> things. Sometimes it is native interop, but sometimes it is for 
> serializing out to a known binary file format, etc. In the absence of 
> these extra instances we do not have to choose its "real" role. If we 
> added them then we'd start committing to one of those scenarios.

The Storable class is part of the FFI and thus I think we must comply to 
the system ABI.