SPECIALIZE pragma

[Ross Paterson]

Digging around in the source code comments, I found a restriction that
is biting me:

> We *insist* that all overloaded type variables are specialised to ground
> types, (and hence there can be no context inside a SPECIALIZE pragma).

The latter is fine, but the former seems too restrictive.  For example,
a function with constraints (Storable a, Eq a) can't be specialized to
Ptr b, even though b would be unconstrained.  Any hope of relaxing this?