Swap usize -> ptr transmute for strict_pov API

Removes some unsafety and reduces the number of `usize` -> `ptr`
transmutes which might be helpful for CHERI-like targets in the future.

library/alloc/src/raw_vec/mod.rs

@@ -410,8 +410,7 @@ unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
 impl<A: Allocator> RawVecInner<A> {
     #[inline]
     const fn new_in(alloc: A, align: Alignment) -> Self {
-        // SAFETY: `Alignment` is non-zero.
-        let ptr = unsafe { core::mem::transmute(align) };
+        let ptr = Unique::from_non_null(NonNull::without_provenance(align.as_nonzero()));
         // `cap: 0` means "unallocated". zero-sized types are ignored.
         Self { ptr, cap: ZERO_CAP, alloc }
     }

library/alloctests/lib.rs

@@ -28,6 +28,7 @@
 #![feature(iter_next_chunk)]
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array_transpose)]
+#![feature(nonnull_provenance)]
 #![feature(ptr_alignment_type)]
 #![feature(ptr_internals)]
 #![feature(sized_type_properties)]