WIP: Bitarray backed Int EAs
First pass at #21839 and most likely a precursor to #22226.

Very hacky at the moment but was able to get tests to pass locally (will fail CI given new Bitarray depdencendy). Going to work through a refactor of this but sharing as POC in case anyone has feedback.

Generally the hard part here was working around places in the code base that expect an ndarray (ex: via `sum` method calls) and figuring out how / when to best manage the conversions between bit arrays and ndarrays