Rollup merge of #154925 - zachs18:heap-into-array-with-alloc, r=Mark-Simulacrum

Make Box/Rc/Arc::into_array allocator-aware (and add doctest)

Tracking issue for `alloc_slice_into_array`: https://github.com/rust-lang/rust/issues/148082

Tracking issue for `allocator_api`: https://github.com/rust-lang/rust/issues/32838

Make the `into_array` methods on `Box`, `Rc`, and `Arc` allocator-aware.

I think the allocator-aware-ness should not be observable on stable, so these should not be additionally (comment-)gated behind `feature(allocator_api)`, just like e.g. `Box::leak` and `Vec::into_boxed_slice`. The added doctests do not use `feature(allocator_api)`.

@rustbot label T-libs-api