Addressing a Share `static mut` should be safe

[alexcrichton]

We talked about this at the most recent workweek, but I believe this code should compile as-is:

use std::sync::atomics;

static mut CNT: atomics::AtomicUint = atomics::INIT_ATOMIC_UINT;

fn main() {
    CNT.fetch_add(1, atomics::SeqCst);
}

Specifically, you should be allowed to take the address (&-pointer) of a static mut, and you should continue to need unsafe to take a &mut-pointer to a static mut.

[alexcrichton]

cc @flaper87

[alexcrichton]

Nominating.

[flaper87]

Makes sense to me! Assigning myself to this issue.

[pnkfelix]

Marking P-low, not on 1.0 milestone. (We can add this backwards compatibly, at least if one ignores lints.)