Completely refactors layout dispatchers by switching from a class-based approach to one that leverages pure functions. While the logic itself isn't any simpler, it was easier to implement, and now hopefully understand, correctly. This conversion was motivated by several bugs that had cropped up related to improper usage of anyio.

Issues Fixed:

• Stale event handlers with frequent state updates #330
• IDOM causes race condition in AnyIO #298

Highlighted Commits:

• improve docs + simplify multiview - 4129b60
• require anyio>=3.0 - 24aed28
• refactor dispatchers - ce8e060