Extracts only send rights from the given mailbox, returning a (possibly shared) handle to the underlying mailbox containing only send rights. Messages sent to mbox go to the underlying mailbox, but messages may not be received from mbox. (Attempting to do so signals a <only-send-rights> condition.)
Note: This is implemented using a proxy wrapper class for mailboxes. The send-message! primitive knows how to indirect through the proxy wrapper.
Note: This is not currently implemented.