Add orders.msg_sender()

[MartinKavik]

It'll reduce boilerplate required for sending messages from JS callbacks.

See master...MartinKavik:demo/websocket_binary
We should mirror core changes from that commit and add comments + perhaps also add notes to orders.clone_app() and orders.msg_mapper() that users should use rather orders.msg_sender().

Implementation note: Use app.update or app.mailbox() + .send(..)?