Models & varinfos part 2

#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