mutations.py has historically contained both the mutations and the

mock models that it creates for simulation. This led to an increasingly

large file. This change moves the mock model support out into its own

file, to help keep the code organized.
There were a few small changes made to MockMeta in the process, just

for code cleanup. Some of the old functions that were later wrapped as

properties are now just property functions. setup_fields() pulls

primary_key out early, so that it's not checked repeatedly, and

shuffles some lines to help the code read better. Otherwise, nothing has

really changed.

Unit tests pass.