Make argdiff an optional keyword argument to update

[marcoct]

It would default to the n-tuple of NoChange() values where n is the number of arguments to the generatiev function. Passing in the tuple of NoChange() values is common when doing operations on the top-level generative function of a model in e.g. MCMC.

[marcoct]

I started implementing this here (including making the other arguments excluding the trace itself into keyword arguments): https://github.com/probcomp/Gen/tree/20200410-marcoct-argdiffoptional, but the extra typing for the keyword arguments almost makes the change not worth it.

[marcoct]

Actually, on second thought, we can keep the GFI interface signature the same (no keyword args), but then just the keyword argument wrapper for use by the inference library.

function update(trace;
        args=get_args(trace),
        argdiffs=map((_) -> NoChange(), args),
        constraints=EmptyChoiceMap())
    update(trace, args, argdiffs, constraints)
end

This is indeed what I implemented, so all the extra keyword arguments in calls were unnecessary.

[marcoct]

The same defaults for regenerate should be implemented as part of the same PR.