Add len to mapped memory

vulkan-sys/src/custom/mappedmemory.rs

@@ -29,6 +29,10 @@ impl<'a, T: Clone> VkMappedMemory<'a, T> {
         self.data.clone_from_slice(data);
     }
 
+    pub fn len(&self) -> usize {
+        self.data.len()
+    }
+
     pub fn iter(&self) -> Iter<'_, T> {
         self.data.iter()
     }