Move StructureDefinition from Conformance to Base

[ewoutkramer]

I think by now we have enough experience with our cross-version classes that we introduced in SDK5.x that we can try to move StructureDefinition to Base as well. This will provide lots of opportunities for more shared code, since a lot of our code (like the validator) is using StructureDefinition. Even resource resolvers etc can then be moved to Base. So, I think it is worth trying (success not guaranteed ;-)