Use unchecked construction in Layout::pad_to_align.

Other, similar methods for `Layout` do likewise, and there's already an
`unwrap()` around the result demonstrating the safety.

library/core/src/alloc/layout.rs

@@ -281,7 +281,9 @@ impl Layout {
         // > `usize::MAX`)
         let new_size = self.size() + pad;
 
-        Layout::from_size_align(new_size, self.align()).unwrap()
+        // SAFETY: self.align is already known to be valid and new_size has been
+        // padded already.
+        unsafe { Layout::from_size_align_unchecked(new_size, self.align()) }
     }
 
     /// Creates a layout describing the record for `n` instances of