Understand proposed changes to ip_diffim structure

When he left the project in late 2017, David Reiss was in the middle of a substantial refactoring of the ip_diffim codebase. Review these changes, understand what was being planned, and think about how this relates to the view of ip_diffim you developed on DM-16409.

It's unreasonable to expect you to understand all the background to this work soon after joining the project. After you have some idea of what's going on, please chat to Yusra AlSayyad, Eric Bellm and John Swinbank to get their take on it.

Key tickets to take a look at in this context are:

• DM-5294
• DM-5295
• DM-3704
• DM-11654
• DM-13831

The end goal of this ticket is to decide if we want to invest further work on David's changes, or if we want to move in another direction. Ultimately, that will be a decision that we take jointly in sprint planning, but we'd like you to take this opportunity to develop your own opinions first!