Make it possible to supply chunk size to transfer() & transmit()

[thekid]

Example:

use io\streams\StreamTransfer;

$s= new StreamTransfer($in, $out);
$size= 0;
foreach ($s->transmit(1024) as $transferred) {
  $size+= $transferred;
  Console::write("\r", $size, ' byte(s) transferred');
}
$s->close();

[thekid]

This breaks yielding this inside a web context:

yield from $stream->transmit();

...as the returned numbers are (erroneously) interpreted as delays by the async server before https://github.com/xp-framework/networking/releases/tag/v10.5.1