Optimize `Once::doit`: perform optimistic check that initializtion is
already completed. `load` is much cheaper than `fetch_add` at least
on x86_64.
Verified with this test:
```
static mut o: one::Once = one::ONCE_INIT;
unsafe {
loop {
let start = time::precise_time_ns();
let iters = 50000000u64;
for _ in range(0, iters) {
o.doit(|| { println!("once!"); });
}
let end = time::precise_time_ns();
let ps_per_iter = 1000 * (end - start) / iters;
println!("{} ps per iter", ps_per_iter);
// confuse the optimizer
o.doit(|| { println!("once!"); });
}
}
```
Test executed on Mac, Intel Core i7 2GHz. Result is:
* 20ns per iteration without patch
* 4ns per iteration with this patch applied
Once.doit could be even faster (800ps per iteration), if `doit` function
was split into a pair of `doit`/`doit_slow`, and `doit` marked as
`#[inline]` like this:
```
#[inline(always)]
pub fn doit(&self, f: ||) {
if self.cnt.load(atomics::SeqCst) < 0 {
return
}
self.doit_slow(f);
}
fn doit_slow(&self, f: ||) { ... }
```
Similarly to the rest of the previous commits, this moves the once primitive to
using &self instead of &mut self for proper sharing among many threads now.
