Towards better treatment of modules and module types.

[cristianoc]

Fixes #178

[cristianoc]

This is a first step towards better treatment of modules and module types. The whole area is in need of some love.
For now, introduce a Constraint(M, MT) to represent the constraint that module M has module type MT.

[cristianoc]

Tests work as expected now.
There's still the pre-existing issue of resolving transitive module references.