RFC: First class support for promises, async/await

Adds first class support for reading the result of a JavaScript Promise
using Suspense:

- Introduces support for async/await in Server Components. Write Server
  Components using standard JavaScript `await` syntax by defining your
  component as an async function.
- Introduces the `use` Hook. Like `await`, `use` unwraps the value of a
  promise, but it can be used inside normal components and Hooks,
  including on the client.

This enables React developers to access arbitrary asynchronous data
sources with Suspense via a stable API.