stream: create the stream/iter share() read reaction once per consumer

share()'s #readAfterPull() created each consumer's afterPull() reaction
lazily with `state.afterPull ??= () => {...}`. Because the arrow function
captures `this` and `state`, V8 allocates its context on every call of
#readAfterPull(), even when afterPull() already exists: about 65 bytes
per read that waits for the source.

afterPull() is now created by #createAfterPull(), so the context is only
allocated when it is.

share() fan-out, async source, two consumers, 16-byte chunks: 2065 ->
1944 bytes allocated per chunk.

Assisted-by: OpenCode

Author: jasnell

lib/internal/streams/iter/share.js

@@ -408,7 +408,15 @@ class ShareImpl {
   // longer pending: clear it from pendingNext, unless a next() has been
   // queued behind it, without a reaction of its own.
   #readAfterPull(state) {
-    state.afterPull ??= () => {
+    // afterPull() is created by a method of its own: if it were created
+    // here, V8 would allocate the context it captures on every call, even
+    // when it already exists.
+    state.afterPull ??= this.#createAfterPull(state);
+    return PromisePrototypeThen(this.#pullFromSource(false), state.afterPull);
+  }
+
+  #createAfterPull(state) {
+    return () => {
       let result;
       try {
         result = this.#tryRead(state);
@@ -426,7 +434,6 @@ class ShareImpl {
       this.#clearWaitingRead(state);
       return result;
     };
-    return PromisePrototypeThen(this.#pullFromSource(false), state.afterPull);
   }
 
   #clearWaitingRead(state) {