pub fn copy(&mut self, item: &T) {
        // TODO: Some types `T` should just be cloned.
        // E.g. types that are `Copy` or vecs of ZSTs.
        unsafe {
            self.local.push(self.inner.copy(item));
        }
    }
Copies an element in to the region.

The element can be read by indexing
differential-dataflow/src/containers.rs