Turns a Reader, r, into an async iterator.

 let f = await Deno.open("/etc/passwd");
 for await (const chunk of Deno.iter(f)) {
   console.log(chunk);
 }
 f.close();

Second argument can be used to tune size of a buffer. Default size of the buffer is 32kB.

 let f = await Deno.open("/etc/passwd");
 const iter = Deno.iter(f, {
   bufSize: 1024 * 1024
 });
 for await (const chunk of iter) {
   console.log(chunk);
 }
 f.close();

Iterator uses an internal buffer of fixed size for efficiency; it returns a view on that buffer on each iteration. It is therefore caller's responsibility to copy contents of the buffer if needed; otherwise the next iteration will overwrite contents of previously returned chunk.