tests from Sean, modified to use rackunit
more consistent: dist-modes
work on dist docs
more doc updates
observe-at w/ continuous vars for rejection-sampler

Not totally sure this technique is right.
extended entry w/ ll field
... track ll-diff, nchoices incrementally
use incrementally-computed ll-diff for MH
update todo
clean up db-stochastic-ctx

Use run method and prompt instead of escape continuation.
That makes it easier to separate ctx creation and use.
MH perturb creates delta-db rather than mutating
eliminate bad MH reject & threshold modes