Fix bugs in proposer calculation post-Fulu (#8101)

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>

consensus/state_processing/src/epoch_cache.rs

@@ -123,7 +123,7 @@ pub fn is_epoch_cache_initialized<E: EthSpec>(
     let current_epoch = state.current_epoch();
     let epoch_cache: &EpochCache = state.epoch_cache();
     let decision_block_root = state
-        .proposer_shuffling_decision_root(Hash256::zero())
+        .epoch_cache_decision_root(Hash256::zero())
         .map_err(EpochCacheError::BeaconState)?;
 
     Ok(epoch_cache
@@ -146,7 +146,7 @@ pub fn initialize_epoch_cache<E: EthSpec>(
     let current_epoch = state.current_epoch();
     let next_epoch = state.next_epoch().map_err(EpochCacheError::BeaconState)?;
     let decision_block_root = state
-        .proposer_shuffling_decision_root(Hash256::zero())
+        .epoch_cache_decision_root(Hash256::zero())
         .map_err(EpochCacheError::BeaconState)?;
 
     state.build_total_active_balance_cache(spec)?;