This introduces the following new functions in

django_evolution.utils.models:


walk_model_tree(): Iterates through a model an all of its parents.


get_model_rel_tree(): Builds and returns a cached dictionary mapping

  table names to reverse relations.


clear_model_rel_tree(): Clears the cached relation tree state.


iter_model_fields(): Iterates through fields on a model, yielding

  fields based on the provided criteria.


iter_non_m2m_reverse_relations(): Iterate through all actual reverse

  relation fields that point to another field (excluding

ManyToManyField fields but including fields on their "through"

  tables).


Each of these are basically equivalent to functionality internal to

modern versions of Django, but lacking any equivalents in earlier

versions. They'll be crucial for upcoming work on field type changes.

Unit tests pass on all supported versions of Django.
Made use of this new functionality in the in-progress field type changes.