Proposal: Remove Show and Eq superclasses of Num

[Paterson, Ross]

Sebastian Fischer writes:
> Then Eq and Show must be superclasses of Num forever, which is the
> wart Ian wants to address. I think that these superclasses should be
> removed *and* the classes should be restructured along the lines of
> yap. I don't care in which order, so +1 from me.

If we had a Ring class, the superclasses of Num would be of much less
consequence.

We could do both, but they each have significant costs in breakage, which
would need to be justified.  If we're considering breaking changes to
the basic Haskell 98 classes, we should examine the range of options and
work out which gives the best value in terms of benefits over breakage.