Rollup merge of #156160 - kx0101:cow-vec-symmetric-partialeq, r=Mark-Simulacrum

feat: add symmetric PartialEq impls for Vec, &[T], &mut [T] versus Cow<'_, [T]>

add the missing reverse `PartialEq<Cow<'_, [U]>>` impls for `Vec<T, A>`, `&[T]`, and `&mut [T]`, essentially mirroring the existing forwards in `library/alloc/src/vec/partial_eq.rs`

partially addresses rust-lang/rust#152830. The `VecDeque` half of that issue is being handled separately by rust-lang/rust#152972, so there is no overlap with this PR

also fyi: verified locally with `./x test library/alloctests --stage 1` and the new `test_partial_eq_cow_symmetric` test passes alongside the existing alloc test suite