Proposal: instance TestEquality IORef

[Ashley Yakeley]

The base package includes the class Data.Type.Equality.TestEquality, for 
types from which type equality can be deduced from values.

Given values ra rb of types "IORef a" and "IORef b", if they are the 
same value (i.e. ra == unsafeCoerce rb), then one can deduce "a ~ b". 
Therefore, IORef can be made an instance of TestEquality. The same is 
true for various other reference types.

I propose these instances in base:

   instance TestEquality IORef
   instance TestEquality (STRef s)

-- Ashley Yakeley