Enforce correct types for channels

[eholk]

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.

[msullivan]

What's the problem here?

[eholk]

I think the issue is that we're sending a @int, which could be shared mutable state. I'm guessing with move semantics, this is actually legal, since typestate and such will make sure we don't actually share it. I'll go ahead and close this.