The Diff class is responsible for taking two project signatures and

diffing them, showing the changes that the target signature has that the

original one does not, and generating a hinted evolution to turn the

original into the target. It's old code that was getting harder to

reason about and maintain.
This change fixes up that code a bit, adding and clarifying

documentation and separating out information gathering from state storage

for the diff (and making it far more efficient in the process). It's now

easier to read through and see the various bits of logic and to modify

it. This will be needed for the upcoming signature work.

Unit tests pass for all versions of Django.