Add an iterator for receiving messages from GenericPorts

[brendanzab]

I've noticed I use this pattern quite a bit:

do spawn {
    loop {
        match port.try_recv() {
            Some(x) => ...,
            None => ...,
        }
    }
}

The RecvIterator, returned from a default recv_iter method on the GenericPort trait, allows you to reduce this down to:

do spawn {
    for x in port.recv_iter() {
        ...
    }
}

As demonstrated in the tests, you can also access the port from within the for block for further recving and peeking with no borrow errors, which is quite nice.