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.

[robambalu]

this diff is hard to read. I think it was moved around to account for duplicate field names being allowed now, but can you explain what the actual diff was in this re-org

[AdamGlustein]

Before, we would create the derived struct field masks for this level of struct before we process the base class fields
Now, since we may need to substitute a derived struct field into a base class field slot, we create the masks after

In the first section (adding in base fields), nothing changed until the loop on line 121 where before we would throw on a name collision, and now we check to see if its a valid override.
In the second (creating the masks), nothing changed except an index shift as we are iterating on the derived fields at the end of m_fields now, not the beginning