Let mutable userdata references in functions

src/userdata.rs

@@ -288,3 +288,10 @@ impl<'a, T: Userdata> FromLua<'a> for &'a T {
         ud.as_ref::<T>()
     }
 }
+
+impl<'a, T: Userdata> FromLua<'a> for &'a mut T {
+    fn from_lua(_lua: &'a State, val: Value<'a>) -> crate::Result<Self> {
+        let ud = val.into_userdata()?;
+        ud.as_mut::<T>()
+    }
+}