Auto merge of #139430 - scottmcm:polymorphic-array-into-iter, r=cuviper

Polymorphize `array::IntoIter`'s iterator impl

Today we emit all the iterator methods for every different array width.  That's wasteful since the actual array length never even comes into it -- the indices used are from the separate `alive: IndexRange` field, not even the `N` const param.

This PR switches things so that an `array::IntoIter<T, N>` stores a `PolymorphicIter<[MaybeUninit<T>; N]>`, which we *unsize* to `PolymorphicIter<[MaybeUninit<T>]>` and call methods on that non-`Sized` type for all the iterator methods.

That also necessarily makes the layout consistent between the different lengths of arrays, because of the unsizing.  Compare that to today <https://rust.godbolt.org/z/Prb4xMPrb>, where different widths can't even be deduped because the offset to the indices is different for different array widths.

Author: bors