Add support for binding a method off an impl

[lkuper]

Make it legal to do the following:

obj foo() {
    fn add5(n: int) -> int {
        ret n + 5;
    }
}

fn add5(n: int) -> int {
    ret n + 5;
}

fn main() {
    let fiveplusseven = bind add5(7);
    log_err fiveplusseven();

    // Can't do this currently
    let my_foo = foo();
    let fiveplusseven_too = bind my_foo.add5(7);
    log_err fiveplusseven_too();
}

This feature would make it possible to reify methods as standalone functions. (I haven't thought through all the implications of this; I suppose the resulting function would have to capture the object in its scope.)

[lkuper]

Currently, this fails in trans with an "unimplemented re-binding existing function" error.

[msullivan]

With my fix to #754, your program compiles but returns the wrong value.

[msullivan]

This is closely related to #758. One of them probably depends on the other, depending on how we tackle them.

[lkuper]

This particular test is passing now, but it might be accidental. I'm not sure we've got this feature working in general yet; I haven't yet thought through how the latest incarnation of the object system behaves wrt bind.

[msullivan]

I think it working is basically accidental? I suspect that if the object had fields things would be a bit more problematic.

[lkuper]

I'm unassigning myself from this one and marking it as unassigned, since, realistically speaking, I don't think I'll have time to tackle it in the next four days.

[marijnh]

This was working only by accident (the resulting closure doesn't actually hold on to the object, so it segfaults when called after the object dies). I assumed the closed over thing was the object box, but in fact it's a {vtbl_, box_} pair. Extra machinery will be needed to wrap this in a closure.