Cache rework

[kesne]

Right now the cache is global, which works well, and even works for when providers change. However, this doesn't make a lot of sense long-term, because it causes unbounded cache growth, and resetting the provider doesn't actually free any memory.

I think there's a couple ways to solve this:

• Cache object is provided by the provider itself. This should help clean up memory when providers are reset / removed.
• Cache is a user-configurable LRU cache, with the ability to explicitly retain data that is actively being used.
• Hooks cause data being used to be explicitly retained (need to figure out how this works with suspense and concurrent).