An attempt at making the accept-request interface safe again

[laumann]

Fixes #8

[laumann]

With these changes the example from #8 is correctly rejected.

extern crate rust_sessions;

use std::thread::spawn;
use std::sync::mpsc::channel;

use rust_sessions::*;

type Proto = Send<u8, Eps>;

fn main() {
    let (tx, rx) = channel();
    let guard = spawn(|| {
        let c: Chan<(), Proto> = accept(tx).unwrap();
        c.send(42).close();
    });
    let c: Chan<(), Proto> = rx.recv().ok().unwrap(); // Does not use request!!
    c.send(42).close();
}

produces the following error:

examples/fail.rs:16:30: 16:53 error: mismatched types:
 expected `rust_sessions::Chan<(), rust_sessions::Send<u8, rust_sessions::Eps>>`,
    found `rust_sessions::Chan<(), rust_sessions::Recv<u8, rust_sessions::Eps>>`
(expected struct `rust_sessions::Send`,
    found struct `rust_sessions::Recv`) [E0308]
examples/fail.rs:16     let c: Chan<(), Proto> = rx.recv().ok().unwrap();
                                                 ^~~~~~~~~~~~~~~~~~~~~~~

The problem seems to be that request was transmuting the received channel Chan<P>. But the accept() function should be sending the value Chan<P::Dual> instead.

[Munksgaard]

@laumann: Let's merge this?