[Cython] #303 + simplifying type system

[Dag Sverre Seljebotn]

Robert Bradshaw wrote:
> I think we should be using sizeof(T) as often as possible to do the
> right thing, but we should still keep track of the rank. I do think
> the current type system could be simplified without getting rid of
> the rank though.

But currently there is just no way to reliably compare the ranks of
external typedefs; and if we introduced that requirement, there's still no
way we could compare them with the ranks of non-external types.

Dag Sverre