rename Generator to Coroutine

[viruscamp]

follow rename Generator to Coroutine and fix #10

add support for immovable coroutine (self referenced)

let mut g = gen_iter!(static {
    let v1 = [1, 2];
    let v = &v1;
    for i in 0..v.len() {
        yield v[i];
    }
});

assert_eq!(g.next(), Some(1));
assert_eq!(g.next(), Some(2));
assert_eq!(g.next(), None);