Change `NonNull::{as_ref, as_mut}` to take `self` rather than `&self` in a future edition

[tgross35]

NonNull has two methods that take &self even though the type is Copy:

• as_ref
• as_mut

The raw pointer methods take self so it is surprising NonNull is different. Taking a reference is unnecessary here and the returned lifetime is unrelated to it.

If we ever get a good way to give methods edition-dependent signatures, this may be something worth considering. Inspired by @cuviper's comment at https://rust-lang.zulipchat.com/#narrow/channel/136281-t-opsem/topic/Completing.20our.20set.20of.20pointer-conversion.20methods/near/423274344