The Arrow spec has a "run-end encoded layout" (https://arrow.apache.org/docs/format/Columnar.html#run-end-encoded-layout) but it doesn't appear to be supported in Arrow.jl. Seems fairly straightforward to implement since the run_ends and values arrays are "child arrays" and not buffers, so we could "just" re-use teh existing functionality to represent the values and run-ends.