Efficient Field Polymorphism

[gilbo]

The initial proposal for field polymorphism ( #34 ) suggests fixing field names/identities via typechecking. This is fine. However, we would eventually like to be able to avoid re-typechecking / re-compiling when field names/identities change but their types/parent relationships remain unchanged.

(As an example, this is an important step to being able to efficiently support temporary fields; since it's not reasonable to re-compile every time the identity of a temporary field changes.)