Address Comments and Cleanup Spec

[ethDreamer]

Biggest change here was just moving the mutation of the state out of get_expected_withdrawals()

[ethDreamer]

Okay I think there was also a bug in process_pending_balance_deposits where it would effectively only function as a rate limiter at the start of the fork and never again. I've updated it based on what I think was the intended logic..

[dapplion]

Thanks for the fixes! process_pending_balance_deposits is easier to read now too