Add diffing support for the new signature classes.
Review Request #10287 — Created Oct. 27, 2018 and submitted
This updates the diffing support to work with the new signature classes.
Diffnow takes these instead of dictionaries. The capabilities and API
hasn't otherwise at this point, but in the future we'll be able to use
the new support to provide more fine-grained diffing so that only
subsets of the signature can be taken into account for evolutions.
Unit tests pass for all supported versions of Python and Django.