Added a very complicated AsyncIterableReader class.

The class was already implemented in
"https://deno.land/std@0.144.0/streams/conversion.ts".  So, I litterally
lost 1 day of work!  I hate myself.

vendor/deno.land/std@0.188.0/io/copy_n.ts

@@ -0,0 +1,41 @@
+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+// This module is browser compatible.
+
+import { assert } from "../_util/asserts.ts";
+import type { Reader, Writer } from "../types.d.ts";
+
+const DEFAULT_BUFFER_SIZE = 32 * 1024;
+
+/**
+ * Copy N size at the most. If read size is lesser than N, then returns nread
+ * @param r Reader
+ * @param dest Writer
+ * @param size Read size
+ */
+export async function copyN(
+  r: Reader,
+  dest: Writer,
+  size: number,
+): Promise<number> {
+  let bytesRead = 0;
+  let buf = new Uint8Array(DEFAULT_BUFFER_SIZE);
+  while (bytesRead < size) {
+    if (size - bytesRead < DEFAULT_BUFFER_SIZE) {
+      buf = new Uint8Array(size - bytesRead);
+    }
+    const result = await r.read(buf);
+    const nread = result ?? 0;
+    bytesRead += nread;
+    if (nread > 0) {
+      let n = 0;
+      while (n < nread) {
+        n += await dest.write(buf.slice(n, nread));
+      }
+      assert(n === nread, "could not write");
+    }
+    if (result === null) {
+      break;
+    }
+  }
+  return bytesRead;
+}