Add new async method Response::transmit() replacing Response::transfer()

[thekid]

The transfer() method will be deprecated (and then removed at a later point!). Existing code will continue to work, but should be rewritten as follows:

-$response->transfer($file->in(), MimeUtil::getByFileName($file->filename), $file->size());
+yield from $response->transmit($file->in(), MimeUtil::getByFileName($file->filename), $file->size());

See also #72 and #70

[thekid]

Released in https://github.com/xp-forge/web/releases/tag/v2.8.0