Stabilize `funnel_shifts` (including `const`)

`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.

Tracking issue: RUST-145686