feat: add qcell

__concurrent/qcell/src/main.rs

@@ -0,0 +1,16 @@
+use std::rc::Rc;
+use qcell::{QCell, QCellOwner};
+
+// https://docs.rs/qcell/latest/qcell/
+fn main() {
+    let mut owner = QCellOwner::new();
+
+    let item = Rc::new(QCell::new(&owner, Vec::<u8>::new()));
+    let iref = owner.rw(&item);
+    // test(&mut owner, &item);    // Compile error
+    iref.push(1);
+
+    fn test(owner: &mut QCellOwner, item: &Rc<QCell<Vec<u8>>>) {
+        owner.rw(&item).push(2);
+    }
+}