layers: Rearchitect Descriptor Set validation code

This change pulls all of the DescriptorSet code out of core_validation.cpp and into
its own files/classes in descriptor_set.h/cpp.

See header file for complete class documentation.

These changes pass tri/cube/smoketest --validate.
All related layer validation tests are also updated and passing.
Finally, I ran it through mustpass CTS and did not hit any issues related to these changes.

These changes not only update the descriptor interface but fix some known lingering
bugs with how descriptor updates occurred. This includes now correctly handling
updates that cross binding boundaries and updates that write a subset of a binding.

Going forward this is a general outline for how we would like to evolve core_validation.
That is, we'd like to move the functionality of the checks into reasonable classes and
just have core_validation call into those classes to do the majority of the work.

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