Represent uniform facts via descriptor set and binding. (#2681)

* Represent uniform facts via descriptor set and binding.

Previously uniform facts were expressed with resepect to the id of a
uniform variable.  Describing them with respect to a descriptor set
and binding is more convenient from the point of view of expressing
facts about a shader without requiring analysis of its SPIR-V.

* Fix equality testing for uniform buffer element descriptors.

The equality test now checks that the lengths of the index vectors
match.  Added a test that exposes the previous omission.