Deeper type resolution via gammaTypes, more errors for datatypes

[dylant-da]

• Check that structs only add optional fields, that variants or enums don't add new variants, and that variants' record types don't add fields at all
• Use Gamma to check types are the same across upgrades
• Add a MonadGammaF so that we can have two different world states (the past and the present) for type synonym resolution in a single monad.