pub unsafe fn local(&mut self) -> &mut [T] {
        &mut self.local[..]
    }
Unsafe access to `local` data. The slices stor data that is backed by a region
allocation. Therefore, it is undefined behavior to mutate elements of the `local` slice.

# Safety
Elements within `local` can be reordered, but not mutated, removed and/or dropped.
differential-dataflow/src/containers.rs