Add Structs to Dictionary Thrift

This is almost certainly not quite right yet, although trying to do the _bare minimum_ to support structs in Dictionary. This PR doesn't allow for import, and thus I have punted on places where the struct is actually used.

EDIT: I haven't done anything about `List` yet - will add separately.
For "right thing" now is good with code, I know it feels a bit clumsy but I think it will end up being a lot better if we just create separate types.