Typechecker doesn't propagate collection item type in for [each]

[marijnh]

Both of these loops don't typecheck, although it should be deducable what the type of u is:

iter x() -> int {
    put 1;
    put 2;
}

fn main() {
    for each u in x() {
        log_err u == 1;
    }
    for u in ~[1, 2, 3] {
        log_err u == 2;
    }
}

[marijnh]

(Interestingly, the code looks like it is doing the right thing in this case.)