Rollup merge of #72414 - KodrAus:feat/stdlazy, r=Mark-Simulacrum

 Add lazy initialization primitives to std

Follow-up to #68198

Current RFC: https://github.com/rust-lang/rfcs/pull/2788

Rebased and fixed up a few of the dangling comments. Some notes carried over from the previous PR:

- [ ] Naming. I'm ok to just roll with the `Sync` prefix like `SyncLazy` for now, but [have a personal preference for `Atomic`](https://github.com/rust-lang/rfcs/pull/2788#issuecomment-574466983) like `AtomicLazy`.
- [x] [Poisoning](https://github.com/rust-lang/rfcs/pull/2788#discussion_r366725768). It seems like there's [some regret around poisoning in other `std::sync` types that we might want to just avoid upfront for `std::lazy`, especially if that would align with a future `std::mutex` that doesn't poison](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/parking_lot.3A.3AMutex.20in.20std/near/190331199). Personally, if we're adding these types to `std::lazy` instead of `std::sync`, I'd be on-board with not worrying about poisoning in `std::lazy`, and potentially deprecating `std::sync::Once` and `lazy_static` in favour of `std::lazy` down the track if it's possible, rather than attempting to replicate their behavior. cc @Amanieu @sfackler.
- [ ] [Consider making`SyncOnceCell::get` blocking](https://github.com/matklad/once_cell/pull/92). There doesn't seem to be consensus in the linked PR on whether or not that's strictly better than the non-blocking variant.

In general, none of these seem to be really blocking an initial unstable merge, so we could possibly kick off a FCP if y'all are happy?

cc @matklad @pitdicker have I missed anything, or were there any other considerations that have come up since we last looked at this?

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