General support for random variables defined with `Scan`

[rlouf]

Tasks

Beta Give feedback

1. Scan logprob fails when not all RVs are valued #135
   bug op-probability scan +3
2. Value transforms not working with Scan #107
   bug rv-transforms scan +3
3. Support "transitive" Scan log-probabilities #75
   enhancement graph rewriting help wanted important op-probability scan +6