Use zero for initialized Once state By re-labeling which integer represents which internal state for `Once` we can ensure that the initialized state is the all-zero state. This is beneficial because some CPU architectures (such as Arm) have specialized instructions to specifically branch on non-zero, and checking for the initialized state is by far the most important operation. As an example, take this: ```rust use std::sync::atomic::{AtomicU32, Ordering}; const INIT: u32 = 3; #[inline(never)] #[cold] pub fn slow(state: &AtomicU32) { state.store(INIT, Ordering::Release); } pub fn ensure_init(state: &AtomicU32) { if state.load(Ordering::Acquire) != INIT { slow(state) } } ``` If `INIT` is 3 (as is currently the state for `Once`), we see the following assembly on `aarch64-apple-darwin`: ```asm example::ensure_init::h332061368366e313: ldapr w8, [x0] cmp w8, #3 b.ne LBB1_2 ret LBB1_2: b example::slow::ha042bd6a4f33724e ``` By changing the `INIT` state to zero we get the following: ```asm example::ensure_init::h332061368366e313: ldapr w8, [x0] cbnz w8, LBB1_2 ret LBB1_2: b example::slow::ha042bd6a4f33724e ``` So this PR saves 1 instruction every time a `LazyLock` gets accessed on platforms such as these.