Refactoring the "moments" design

Previously, we had `RAFTableSection` and `RAFTableSectionMoment` which were essentially the same modulo mutability. We can collapse those into one thing, which results in significant simplification. This also obviates the injection of a custom `RAFTableSectionMoment` subclass, since now users can just use their own `RAFTableSection` subclass directly.

We can also probably do the same for `RAFTableForm` and `RAFTableFormMoment`. Likewise, this will result in injection of custom `RAFTableFormMoment` subclasses becoming unnecessary.

The trick is that each of these things should have a mutable and an immutable interface, and they should project a signal of their "moments", i.e. their immutable states at a single moment in time.