Cache individual element paints between renders

[JoshKarpel]

If a component hasn't changed (see #72, which this probably depends on), then we know that the paints of the elements bounded by that component haven't changed either.

• There is one other thing (other than state changes) that can break this cache: terminal resize events!
• It seems like the cache needs to live in the shadow tree - I think it is one node per element now, not per component, so that should be straightforward.
• Not quite sure how to get this information to the painter. Right now it takes a concrete element tree. Should it be inside that data structure, or somehow adjacent? I guess we could use id() as a cache key, since if we have Granular state changes #72 we'd be caching the actual produced element instances.
• Now that I'm thinking about it, its a little odd to cache at this layer but not at the previous layer (layout). Caching layout is a little tricky because elements can potentially take imperative actions on their parent's layout... does that break this idea? Need to be careful here.