Rewrite sync::one::Once.doit

The old Once.doit() was overly strict and used read-modify-writes when
it didn't have to. The new algorithm will be just a read-acquire load in
the already-initialized case, and the first-call and contested cases use
less strict memory orderings than the old algorithm's SeqCst.

On my machine (3.4GHz Intel Core i7) I was seeing the old algorithm take
13ns (in the already-initialized case). The new one takes only 3ns, and
that drops to 0.5ns with the #[inline] annotation. I am unsure how to
properly measure the contested case, because of the very nature of this
object.

This algorithm definitely needs to be carefully reviewed by someone with
more experience using atomics.