This should make using Reagent with figwheel more convenient (no need
for force-update-all). Add a fourth argument to render that controls
the updating.
This makes most comparisons faster, and should result in fewer
re-renderings. And it should be a lot easier to explain and
reason about than the old heuristics.
It also makes it much easier to make custom objects, e.g. different
cursor implementations, avoid unnecessary re-renderings.
The main downside is that it is no longer possible to pass infinite
sequences to components.