[Scheduler] Store Tasks on a Min Binary Heap (facebook#16245)

* [Scheduler] Store Tasks on a Min Binary Heap

Switches Scheduler's priority queue implementation (for both tasks and
timers) to an array-based min binary heap.

This replaces the naive linked-list implementation that was left over
from the queue we once used to schedule React roots. A list was arguably
fine when it was only used for roots, since the total number of roots is
usually small, and is only 1 in the common case of a single-page app.

Since Scheduler is now used for many types of JavaScript tasks (e.g.
including timers), the total number of tasks can be much larger.

Binary heaps are the standard way to implement priority queues.
Insertion is O(1) in the average case (append to the end) and O(log n)
in the worst. Deletion is O(log n). Peek is O(1).

* Sophie nits

Author: acdlite

packages/react-dom/src/__tests__/ReactUpdates-test.js

@@ -1631,6 +1631,7 @@ describe('ReactUpdates', () => {
             ReactDOM.render(<App />, container);
             while (error === null) {
               Scheduler.unstable_flushNumberOfYields(1);
+              Scheduler.unstable_clearYields();
             }
             expect(error).toContain('Warning: Maximum update depth exceeded.');
             expect(stack).toContain('in NonTerminating');
@@ -1653,34 +1654,21 @@ describe('ReactUpdates', () => {
         React.useEffect(() => {
           if (step < LIMIT) {
             setStep(x => x + 1);
-            Scheduler.unstable_yieldValue(step);
           }
         });
+        Scheduler.unstable_yieldValue(step);
         return step;
       }
 
       const container = document.createElement('div');
       act(() => {
         ReactDOM.render(<Terminating />, container);
       });
-
-      // Verify we can flush them asynchronously without warning
-      for (let i = 0; i < LIMIT * 2; i++) {
-        Scheduler.unstable_flushNumberOfYields(1);
-      }
       expect(container.textContent).toBe('50');
-
-      // Verify restarting from 0 doesn't cross the limit
       act(() => {
         _setStep(0);
-        // flush once to update the dom
-        Scheduler.unstable_flushNumberOfYields(1);
-        expect(container.textContent).toBe('0');
-        for (let i = 0; i < LIMIT * 2; i++) {
-          Scheduler.unstable_flushNumberOfYields(1);
-        }
-        expect(container.textContent).toBe('50');
       });
+      expect(container.textContent).toBe('50');
     });
 
     it('can have many updates inside useEffect without triggering a warning', () => {