isNull{Fun,}Ptr

[Malcolm Wallace]

> Wolfram Kahl suggested to add functions isNullPtr and
> isNullFunPtr (with the expected semantics) to the Ptr
> module.  Opinions?

Ptr and FunPtr are already instances of Eq, so which is easier to type?
    isNullPtr x
or
    x==nullPtr

So I don't see the motivation for the new predicates, but I don't have
any objections either.

Regards,
    Malcolm