Allow break and others inside do blocks (or stack closures in general)

[mitsuhiko]

Right now you can't break from a loop inside a stack closure. That's counter intuitive. Intended example usage:

    loop {
        do sdl::poll_events |event| {
            match event {
                sdl::QuitEvent => { break; }
                _ => {}
            }
        }
    }

Compiler error currently:

test.rs:11:36: 11:42 error: `break` outside of loop
test.rs:11                 sdl::QuitEvent => { break; }
                                               ^~~~~~

[bblum]

dup of #3064.