More Efficient Validation

[KetanReddy]

Right now for validation we fill in references, solve generics, generate effective types etc. as we walk through the XLR tree. A more efficient way to do things might be to resolve everything in the SDK before passing it to the validation logic. That would probably simplify some steps in validation related to those more expensive operations and if we cache those computed nodes, speed up subsequent validation calls to the same object. Obviously if new types are loaded in we'll need to invalidate that cache but that probably won't happen too often.