This switches all the mutations over to using the new signature classes

instead of the old dictionaries, giving us a standard API for working

with signatures and simplifying a lot of code.
For the most part, the changes are fairly straight-forward moves to the

new API. There are a few larger changes that needed to be made, however.


create_field, MockModel, and MockMeta had to be updated to take

   some additional arguments that were no longer able to be pulled from

   signature dictionaries directly.


SQLMutation now needs to do a bit more work to handle legacy

   mutation functions, as it needs to now serialize the signature to a

   dictionary that the function can operate on and then replace the

   state of the project signature with the results of that function.


ChangeMeta has to do a little more work to check the properties

   being changed and manipualting values, since it can't just operate on

   raw dictionaries anymore. This actually helps make it more readable

   though.


While there's still work that needs to done in some places still to stop

working directly with signature dictionaries, this covers the majority

of those cases. The rest will be handled as part of other changes.

Unit tests pass on all supported versions of Django.