Merge fwddiff_deferred and autodiff_deferred

[wsmoses]

I tried to use autodiff_deferred(Forward, ...) and then realized it errored because there isn't a deferred version that takes the mode.

[vchuravy]

I think I mentioned in the summer that we should try turning autodiff_deferred back into autodiff since we fixed the reason for the split. But the API sufficiently diverged that it wasn't straightforward last time I tried.