Test correctness of gradients end-to-end

[axch]

It should be really easy to get some Venture model, name some scope, and get what the current values there are and what Venture thinks the local posterior gradient there is. This is essentially necessary for testing and debugging gradients, so blocks #383, #385, #386, and #388, not to mention looking for unknown bugs in the AD system. (Some amount of one-SP-at-a-time gradient testing is already being done, but doesn't do a good job of e.g. SPs with auxes -- see #368.)

Ideally, it should also be really easy to evaluate regen multiple times at nearby points with the same random state, to be able to compare the reported gradient to numerically estimated ones. This is #199.