Garbage collection for Versioned models

[misson20000]

We need to keep all versions of a model around that are newer than the oldest one with a live reference to it, regardless of whether those interevening versions have live references to them or not, because things that observe models need to be able to step through changes and those intervening versions one at a time. Currently, we just keep around every version of the model that has ever existed in a big singly-linked list. This is bad and a memory leak. Need to come up with a more clever way to manage this memory.