Add an iterator for receiving messages from GenericPorts

I've noticed I use this pattern quite a bit:

~~~rust
do spawn {
    loop {
        match port.try_recv() {
            Some(x) => ...,
            None => ...,
        }
    }
}
~~~

The `RecvIterator`, returned from a default method on the `GenericPort` trait, allows you to reduce this down to:

~~~rust
do spawn {
    for x in port.recv_iter() {
        ...
    }
}
~~~

As demonstrated in the tests, you can also access the port from within the `for` block with no borrow errors, which is quite nice.

r? @brson