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>

beacon_node/beacon_chain/src/validator_monitor.rs

@@ -497,7 +497,7 @@ impl<E: EthSpec> ValidatorMonitor<E> {
             });
 
         // Add missed non-finalized blocks for the monitored validators
-        self.add_validators_missed_blocks(state);
+        self.add_validators_missed_blocks(state, spec);
         self.process_unaggregated_attestations(state, spec);
 
         // Update metrics for individual validators.
@@ -588,7 +588,7 @@ impl<E: EthSpec> ValidatorMonitor<E> {
     }
 
     /// Add missed non-finalized blocks for the monitored validators
-    fn add_validators_missed_blocks(&mut self, state: &BeaconState<E>) {
+    fn add_validators_missed_blocks(&mut self, state: &BeaconState<E>, spec: &ChainSpec) {
         // Define range variables
         let current_slot = state.slot();
         let current_epoch = current_slot.epoch(E::slots_per_epoch());
@@ -616,8 +616,8 @@ impl<E: EthSpec> ValidatorMonitor<E> {
                 if block_root == prev_block_root {
                     let slot_epoch = slot.epoch(E::slots_per_epoch());
 
-                    if let Ok(shuffling_decision_block) =
-                        state.proposer_shuffling_decision_root_at_epoch(slot_epoch, *block_root)
+                    if let Ok(shuffling_decision_block) = state
+                        .proposer_shuffling_decision_root_at_epoch(slot_epoch, *block_root, spec)
                     {
                         // Update the cache if it has not yet been initialised, or if it is
                         // initialised for a prior epoch. This is an optimisation to avoid bouncing