Use `<Box<[T; N]> as IntoIterator>::IntoIter` to avoid an expensive copy when calling `into_iter` on HeapArray.

[WhyAreAllTheseTaken]

into_iter should use the new implementation in rust-lang/rust#134021 to avoid an expensive array copy.

[WhyAreAllTheseTaken]

As the feature is currently not merged, the implementation in #1 will be used until it is available.