Stabilize funnel_shifts (including const)

[tgross35]

funnel_shl and funnel_shr have been around for close to a year, the unchecked versions for a number of months. These are reasonably small and uncontroversial, and it can be tricky to get similar performance with a fallback; stabilize them here.

Newly stable API:

impl {u8, u16, u32, u64, u128, usize} {
    pub const fn funnel_shl(self, right: Self, shift: u32) -> Self;
    pub const fn funnel_shr(self, right: Self, shift: u32) -> Self;

    pub const unsafe fn unchecked_funnel_shl(self, right: Self, shift: u32) -> Self;
    pub const unsafe fn unchecked_funnel_shr(self, right: Self, shift: u32) -> Self;
}

The tracking issue also mentions a wrapping_ version but it has not been implemented.

Closes: #145686 (tracking issue, wrapping versions will need a new issue)

This involves an intrinsic stabilization so needs FCP from both @rust-lang/lang and @rust-lang/libs-api.

@rustbot label +I-lang-nominated +I-libs-api-nominated

Cc others who have been involved: @RalfJung, @folkertdev, @sayantn

r? libs

[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

⚠️ #[rustc_allow_const_fn_unstable] needs careful audit to avoid accidentally exposing unstable
implementation details on stable.

cc @rust-lang/wg-const-eval

⚠️ #[rustc_intrinsic_const_stable_indirect] controls whether intrinsics can be exposed to stable const
code; adding it needs t-lang approval.

cc @rust-lang/wg-const-eval

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

[tgross35]

Note that const_trait_impl is only needed for the fallback. We could implement a version without it if needed.

The fallback traits are gated behind core_intrinsics_fallbacks, I'm not sure whether there's a cleaner way to do this.

View changes since the review

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.