Models & varinfos part 2

[penelopeysm]

#660 covers the high-level interface of how to work with models and VarInfos. This would be a follow-up post, which discusses the details about how this actually works implemented.

• __varinfo__
• how models + VarInfo interact: tilde_assume!! and tilde_observe!! (the 'tilde pipeline' which isn't really a pipeline any more)
• leaf contexts
• accumulators