New reconciler

[LPGhatguy]

The reconciler is starting to get pretty darn big, complicated, and a bit convoluted.

I'd like to take another stab at rewriting it from the ground up with better-defined types, and to be a little bit more robust.

My wishlist for the new reconciler:

• Asynchronous Rendering
• Fragments
• Better error handling, resolving Wrap lifecycle hooks in NoYield blocks? #55 and Errors put Roact in a bad state #84
• New 'tree root' object, resolving Globally expose instance handles #89 and Signal when a component is rendered #90
• Foundation for new context API, from New Context API #4
• Refactored portals
• Better setState behavior, resolving setState locking throws confusing errors from event listeners #56 and Suspend event listeners during reconciliation? #71
• Support for multiple/alternative reconcilers, for out-of-Roblox use and unit testing
• Complete test coverage!

[AmaranthineCodices]

One idea I keep coming back to related to this: what if we had multiple reconcilers? - or at least the capability to support multiple reconcilers?

It'd be really cool to have:

• ShallowReconciler, like React's ShallowRenderer - this is used for unit testing. It renders only a component's root return value and its immediate children, and no further.
• TestReconciler, like React's TestRenderer - this is also used for unit testing. It completely renders a component tree to a series of objects.

[LPGhatguy]

I updated the issue with a complete wishlist of the things I'm looking for in the new reconciler.