feat: shared view pools [wip]

[rigor789]

This is a draft/wip branch of implementing shared view pools.

Shared pools will allow defining the item templates in a separate view/element, and reuse them for multiple CollectionViews. This is especially useful for nested CollectionViews, where we may have multiple CollectionViews rendering the same templates.

For simplicity, pool related logic has been moved to a new SharedCollectionViewPool class, so even if the view doesn't share the pool, it will still create one automatically and use that (rather than duplicating the logic in both classes).

todos:

• handle unloading/re-loading the view
• thoroughly test to make sure recycling behaves correctly
• implement iOS (at least supporting using templates from the SharedCollectionViewPool
• refine api, property naming
• add demos
• add framework wrappers
• document usage