create `PnpInferenceState` with logprob and scores, implement (one-shot) sampling

[joelgrus]

Create new object PnpInferenceContext that contains

• the CompGraph
• a set of active ExecutionScores

and change everything to use it.

Also, the one-shot sampling should be working correctly. After this PR I'll take a stab at the particle-filter version.

[joelgrus]

ok, I think this version is more what you had in mind, let me know how you feel about it