Granular state changes

[JoshKarpel]

Right now, when we detect a state change, we re-run the entire component tree. This is overkill - we can know which component the state changed in, and re-run only that component and its descendants.

• This should result in some speedup because we'll be running less code overall, especially potentially-expensive things in components that we know didn't change at all.
• We'll need some way to mark the dirty shadow node, and then to propagate that dirtiness to its children.