`PrivateMethods`' items aren't private, they are accessible via `MaskElement`

A surprising new SemVer breakage case was discovered earlier this week, affecting sealed traits' associated items (consts, types, functions with or without receivers — everything) being accessible to subtraits. This is non-obvious and usually isn't intended by the authors of such code, which can lead to accidental breakage.

portable-simd has such an at-risk pattern here

mod private_methods {
    pub impl(super) trait PrivateMethods {
        fn valid<const N: usize>(values: Simd<Self, N>) -> bool
        where
            Self: SimdElement;

        fn eq(self, other: Self) -> bool;

        fn to_usize(self) -> usize;
        fn max_unsigned() -> u64;

        type Unsigned: SimdElement;

        const TRUE: Self;

        const FALSE: Self;
    }
}

Despite the name PrivateMethods, all of those items and methods are accessible downstream (including in downstream crates) via any pub subtrait of PrivateMethods, such as the adjacent MaskElement:

pub impl(self) unsafe trait MaskElement:
    SimdElement<Mask = Self> + SimdCast + PrivateMethods {}

Stabilizing MaskElement could lead to accidentally stabilizing all those PrivateMethods items too, which seems undesirable.

This is a sibling issue of https://github.com/rust-lang/rust/issues/158654 which flags an analogous concern in the Rust standard library itself.

Hat tip to @jhpratt for looping me in to look at the SemVer breakage and suggesting that we scan portable-simd for this pattern. I used AI tools for the scan, but wrote this issue fully by hand otherwise.

Start in crates/core_simd/src/masks.rs, reading the private_methods::PrivateMethods trait and the adjacent public MaskElement trait. Check how each associated item is exposed through the subtrait and verify the intended visibility behavior, including from a downstream crate if possible. Done means the PrivateMethods items are no longer unintentionally accessible through MaskElement.