Add user space messaging

[Thomasdezeeuw]

This add three new functions to SubmissionQueue:

• receive_msg: creates a MsgListener to listen for messages.
• try_send_msg: send a message to the MsgListener.
• send_msg: same as try_send_msg, but retries if the submission queue is full (returns a Future).

It also adds a new module called msg, with the following types:

• MsgListener: listener returned by SubmissionQueue::receive_msg.
• SendMsg: future behind SubmissionQueue::send_msg.
• MsgToken: token used to send a message to a specific listener.