Various clean-ups simplifications for the fork-choice / proto-array crate. They were motivated while working on https://github.com/sigp/lighthouse/issues/7089 (non-finalized checkpoint sync). - https://github.com/dapplion/lighthouse/pull/48 - https://github.com/dapplion/lighthouse/pull/49 - https://github.com/sigp/lighthouse/pull/8324 - https://github.com/dapplion/lighthouse/pull/51 - https://github.com/dapplion/lighthouse/pull/53