python: Add support for partial map and partial set updates

python: Add support for partial map and partial set updates

Allow the python IDL to use mutate operations more freely
by mimicing the partial map and partial set operations now
available in the C IDL.

Unit tests for both of these types of operations are included.
They are not carbon copies of the C tests, because testing
idempotency is a bit difficult for the current python IDL
test harness.

Signed-off-by: Ryan Moats <rmoats@us.ibm.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>