add Storable a => Storable (Ratio a) and Storable a => Storable (Complex a) to base

[Henning Thielemann]

On Mon, 3 Nov 2014, Edward Kmett wrote:

> To be honest I don't think omitting an instance because you don't 
> believe the type should be used at all is a useful strategy.

Explicitly forbidding Storable (Ratio a) has the advantage that we can 
explain the reasons in the documentation of the blocking instance. In 
contrast, a missing instance just looks like someone has forgotten it. It 
would be simple to turn a blocked instance into a useful one in future, 
once one is found. It is also easier to do so if no orphan instances of 
Storable (Ratio a) lie around.