Remove "move" concept by synthesizing reused-removes + adds

[JakeWharton]

With #1902, we will have the ability to remove nodes and add them elsewhere without recreation. Thus, all moves can be synthesized as removes and adds. Since these are exceedingly rare in our experience, it may be worth removing them as a concept altogether.

At the very least, we can make it a default implementation on Widget.Children so that in places where we can efficiently perform moves you still can, but in the case where it's hard or impossible (e.g., our current LazyList design) you don't have to worry about it.