Allow derived structs to override base fields with subtypes

[AdamGlustein]

Closes #517 , allows for derived struct types to specialize fields iff those fields are Python objects or csp.Structs.

For example,

class A(csp.Struct):
    field1: BaseObject
    field2: InnerBaseStruct

class B(A):
    field1: DerivedObject
    field2: InnerDerivedStruct

is allowed, but

class A(csp.Struct):
    field1: int

class B(A):
    field1: float

or

class A(csp.Struct):
    field1: DerivedObject

class B(A):
    field1: BaseObject

are not. Note that a lot of the changes in Struct.cpp is just a reordering of when we create the set/none masks; before we did this before adding base class fields, now we do it after.