fix ArenaPtr payload offset logic

the old logic would be incorrect if the payload has higher alignment than the ArenaHeader i.e. when there is padding between the ArenaHeader and the Payload

src/arena.rs

@@ -283,7 +283,10 @@ pub trait ArenaAllocated {
         Self::Payload: Sized,
     {
         TypedArenaPtr(NonNull::new_unchecked(
-            ptr.payload_offset().cast_mut().cast::<Self::Payload>(),
+            ptr.get_ptr()
+                .byte_add(Self::header_offset_from_payload())
+                .cast_mut()
+                .cast::<Self::Payload>(),
         ))
     }