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

[kx0101]

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 #152830. The VecDeque half of that issue is being handled separately by #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

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r? @Mark-Simulacrum

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[kx0101]

had to update the two UI tests expected compiler-error snapshots so they reflect the new PartialEq<Cow<[U]>>

pretty interesting!