Don't bind expiration time to render callback

This is a fragile pattern because there's only meant to be a single
task per root, running at a single expiration time. Instead of binding
the expiration time to the render task, or closing over it, we should
determine the correct expiration time to work on using fields we
store on the root object itself.

This removes the "return a continuation" pattern from the
`renderRoot` function. Continuation handling is now handled by
the wrapper function, which I've renamed from `runRootCallback` to
`performWorkOnRoot`. That function is merely an entry point to
`renderRoot`, so I've also removed the callback argument.

So to sum up, at at the beginning of each task, `performWorkOnRoot`
determines which expiration time to work on, then calls `renderRoot`.
And before exiting, it checks if it needs to schedule another task.

packages/react-reconciler/src/ReactFiberExpirationTime.js

@@ -21,7 +21,10 @@ import {
 export type ExpirationTime = number;
 
 export const NoWork = 0;
+// TODO: Think of a better name for Never.
 export const Never = 1;
+// TODO: Use the Idle expiration time for idle state updates
+export const Idle = 2;
 export const Sync = MAX_SIGNED_31_BIT_INT;
 export const Batched = Sync - 1;