Review `feedback` for ways to keep minutiae abstracted

[countvajhula]

Similar to how we can introduce a >- subform to -< to "internally" keep track of values and process them in ways that would not be possible externally (without introducing additional, nonlocal, linguistic facilities) (see #128 ), there may be ways to do this for forms like feedback as well, for instance, to make notions like "scratch" and "control" values part of the grammar of the feedback form itself, saving users the trouble of managing these on their own at the low, anonymous level of input indexes. It's possible that the recent addition of bindings to Qi will make this easier to implement internally as well as provide a simple way to refer to these in the feedback definition.