Add examples implementing various PPL styles via Funsor

[fritzo]

@lawmurray suggested adding examples of how to implement different styles of probabilistic programming languages on top of Funsor. We could implement this as a directory examples/ppl-zoo or examples/rosetta or something.

Tasks

(@eb8680 @lawmurray I've tried to populate with representative examples, but I think you both have clearer views of the landscape; feel free to update/add to/remove from this list)

• a Pyro example (minipyro)
• a Birch-style example (with delayed sampling, maybe using pyro.ground)
• a Stan-style example (HMC on a non-normalized log-density function, blocked by Add HMC and NUTS algorithms / interpretations #123)
• a TFP JointDistributionCoroutine-style example wrapping minipyro with yield statements
• an omega/Edward2-style example (with RandomVariables (and RCD?)) Add RandomVariable classes that carry deferred sample calls #130
• an Infer.net-style example using expectation propagation Implement an expectation propagation interpretation #90 or Add moment_matching interpretation + SLDS example #115
• a webPPL-style example (a single Infer operator and basic ADTs and control flow)
• a Dyna/kProbLog-style example (with a polyvariadic fix-point combinator)
• a Church-style example
• a Venture/Gen-style example