assert_once_ever!() macro

[bblum]

Some places in the task code are commented with "only one task should ever do this". Now that we have unsafe global variables, we can implement a safe convenience macro that can assert this at runtime instead of at comment-time.

It should expand to something like:

unsafe {
    static mut __gensymed__thing_already_happened: bool = false;
    // "double-check locking" to avoid doing an xchg all the time
    if __gensymed__thing_already_happened == false {
        if atomic_xchg(&mut __gensymed__thing_already_happened, true, Relaxed) == true {
            fail!($provided_message);
        }
    }
}

[bblum]

I got dis.