Rollup merge of #130822 - bjoernager:non-null-from-ref, r=dtolnay

Add `from_ref` and `from_mut` constructors to `core::ptr::NonNull`.

Relevant tracking issue: #130823

The `core::ptr::NonNull` type should have the convenience constructors `from_ref` and `from_mut` for parity with `core::ptr::from_ref` and `core::ptr::from_mut`.

Although the type in question already implements `From<&T>` and `From<&mut T>`, these new functions also carry the ability to be used in constant expressions (due to not being behind a trait).

Author: matthiaskrgr