Allow nested CachedRenders

[jgaskins]

This fixes #45 by recursively invoking the cache checker if needed, but I want to have a discussion surrounding it before it gets merged in. Specifically, two things I want to discuss:

1. Is this a good idea?
   1. I have no idea what the maintenance burden of this might be. It will probably be trivial, but still worth having a conversation.
   2. It will probably overflow the stack when someone inevitably returns self from render. Do we care?
2. What are the performance implications?
   1. If I'm being completely honest, this is my main concern. I haven't benchmarked this and that makes me nervous. :-)
   2. The most common case will be that the while loop is not invoked, but the conditional, while pretty cheap, isn't free and is potentially invoked thousands of times during a render. Even cheap things can get expensive in loops.
   3. CachedRender is intended to keep our apps fast. If it adds significant overhead, we should decide whether to support this use case.

cc @balmoral