In preparation for some upcoming work on mutators, this change splits up

the somewhat large django_evolution.mutators module into per-class

modules. This will help keep things organized as new mutator classes are

added (which will be needed for the new MoveToDjangoEvolution

mutation).
The actual code in these modules is untouched, with the exception of

the addition of module-level strings, a Version Changed in each class,

and a new docstring for SQLMutator. Docs otherwise are not up to

current standards.

All unit tests pass.
Built the docs and checked for errors.