As identified by a researcher during the Fusaka security competition, we were computing the proposer index incorrectly in some places by computing without lookahead.
- [x] Add "low level" checks to computation functions in `consensus/types` to ensure they error cleanly
- [x] Re-work the determination of proposer shuffling decision roots, which are now fork aware.
- [x] Re-work and simplify the beacon proposer cache to be fork-aware.
- [x] Optimise `with_proposer_cache` to use `OnceCell`.
- [x] All tests passing.
- [x] Resolve all remaining `FIXME(sproul)`s.
- [x] Unit tests for `ProtoBlock::proposer_shuffling_root_for_child_block`.
- [x] End-to-end regression test.
- [x] Test on pre-Fulu network.
- [x] Test on post-Fulu network.
Co-Authored-By: Michael Sproul <michael@sigmaprime.io>
