stream: share webstreams async iterator methods

ReadableStream.prototype.values() built each iterator from an object
literal with a computed symbol-key method plus five closures. Such a
literal is rebuilt through the runtime on every evaluation, costing
close to a microsecond per iterator, which dominates iterating a
short-lived stream.

Move next() and return() to a shared ReadableStreamAsyncIterator
prototype, as for any WebIDL async iterator, and keep the per-iterator
state in its read request. The prototype chain and property shape are
the ones WPT checks; the placeholder AsyncIterator object in util.js is
no longer needed. As in WebIDL, next() and return() now reject when
called on something that is not a ReadableStream async iterator, and
iterators no longer carry own next/return properties.

Add an async-iterator kind to benchmark/webstreams/lifecycle.js.

webstreams/lifecycle.js kind='async-iterator'   ***  +16.22%

Signed-off-by: Matteo Collina <hello@matteocollina.com>
PR-URL: https://github.com/nodejs/node/pull/66392
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>

benchmark/webstreams/lifecycle.js

@@ -9,7 +9,7 @@ const {
 
 const bench = common.createBenchmark(main, {
   n: [5e4],
-  kind: ['readable', 'pipe-to', 'pipe-through'],
+  kind: ['readable', 'async-iterator', 'pipe-to', 'pipe-through'],
 });
 
 const chunk = Buffer.alloc(1024);
@@ -37,6 +37,18 @@ async function readable(n) {
   assert.strictEqual(chunks, n * 4);
 }
 
+async function asyncIterator(n) {
+  let chunks = 0;
+  bench.start();
+  for (let i = 0; i < n; i++) {
+    for await (const chunk of new ReadableStream(makeSource())) {
+      if (chunk) chunks++;
+    }
+  }
+  bench.end(n);
+  assert.strictEqual(chunks, n * 4);
+}
+
 async function pipeTo(n) {
   let chunks = 0;
   bench.start();
@@ -66,6 +78,9 @@ function main({ n, kind }) {
     case 'readable':
       readable(n);
       break;
+    case 'async-iterator':
+      asyncIterator(n);
+      break;
     case 'pipe-to':
       pipeTo(n);
       break;