Add PartialEq<Vec<U>> for [T; N]

[Mandalorian7773]

Summary

This PR fixes asymmetric equality between arrays and Vecs by adding a missing
PartialEq<Vec<U>> for [T; N] implementation.

Previously, comparisons like Vec<T> == [T; N] were supported, but the reverse
[T; N] == Vec<T> failed due to the absence of a corresponding PartialEq impl.
This PR completes the symmetry by forwarding the comparison to existing slice
equality.

Details

• Adds PartialEq<Vec<U>> for [T; N] in the array equality implementation.
• The comparison is implemented by converting both sides to slices and reusing
  existing slice equality logic.
• No new behavior is introduced beyond completing the missing symmetric case.
• No allocations or unsafe code are involved.

Fixes #149017

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 }
 
-
 // NOTE: some less important impls are omitted to reduce code bloat
 // __impl_slice_eq2! { [A; $N], &'b [B; $N] }
 // __impl_slice_eq2! { [A; $N], &'b mut [B; $N] }
fmt: checked 6606 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:00:46
  local time: Thu Dec 18 21:31:05 UTC 2025
  network time: Thu, 18 Dec 2025 21:31:05 GMT

[theemathas]

This is a duplicate of #149045

[Kivooeo]

closing as duplicate