Enforce correct types for channels

This should fail to compile but currently doesn't.

```
fn start(chan[@int] c) {
    c <| @42;
}

fn main() {
    auto p = port();
    auto child = spawn start(chan(p));
    auto c; p |> c;
    log_err *c;
}
```

This will probably be fixed when #568 is fixed.
