Additional caching

[ocrickard]

In previous impls we also implemented the following caching layers in addition to the current bitmap and size cache:

1. Renderer caching. Saves the layout managers, and is commonly lighter weight than the bitmaps that came from them.
2. Attributed string caching. Saves the result of the build steps into a global cache so we avoid re-building attributed strings all the time.

[rnystrom]

Yaaasss. All of it!