Add non-slice Ref constructors w/ slice elem count

Add `Ref` constructors which are generic over `T:
KnownLayout<PointerMetadata = usize>` - in other words, types whose
trailing field is a slice (i.e., slices or slice DSTs). These
constructors take an explicit element count for the trailing slice, and
replace the previous constructors which only supported slices.

Makes progress on #29

src/deprecated.rs

@@ -31,6 +31,7 @@ where
     T: NoCell,
 {
     #[deprecated(since = "0.8.0", note = "`Ref::new_zeroed` now supports slices")]
+    #[must_use]
     #[doc(hidden)]
     #[inline(always)]
     pub fn new_slice_zeroed(bytes: B) -> Option<Ref<B, [T]>> {