Consensus context with proposer index caching (#3604)

## Issue Addressed

Closes https://github.com/sigp/lighthouse/issues/2371

## Proposed Changes

Backport some changes from `tree-states` that remove duplicated calculations of the `proposer_index`.

With this change the proposer index should be calculated only once for each block, and then plumbed through to every place it is required.

## Additional Info

In future I hope to add more data to the consensus context that is cached on a per-epoch basis, like the effective balances of validators and the base rewards.

There are some other changes to remove indexing in tests that were also useful for `tree-states` (the `tree-states` types don't implement `Index`).

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -80,7 +80,7 @@ use state_processing::{
     },
     per_slot_processing,
     state_advance::{complete_state_advance, partial_state_advance},
-    BlockSignatureStrategy, SigVerifiedOp, VerifyBlockRoot, VerifyOperation,
+    BlockSignatureStrategy, ConsensusContext, SigVerifiedOp, VerifyBlockRoot, VerifyOperation,
 };
 use std::cmp::Ordering;
 use std::collections::HashMap;
@@ -3498,7 +3498,6 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         }
 
         let slot = state.slot();
-        let proposer_index = state.get_beacon_proposer_index(state.slot(), &self.spec)? as u64;
 
         let sync_aggregate = if matches!(&state, BeaconState::Base(_)) {
             None
@@ -3645,12 +3644,14 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             ProduceBlockVerification::VerifyRandao => BlockSignatureStrategy::VerifyRandao,
             ProduceBlockVerification::NoVerification => BlockSignatureStrategy::NoVerification,
         };
+        // Use a context without block root or proposer index so that both are checked.
+        let mut ctxt = ConsensusContext::new(block.slot());
         per_block_processing(
             &mut state,
             &block,
-            None,
             signature_strategy,
             VerifyBlockRoot::True,
+            &mut ctxt,
             &self.spec,
         )?;
         drop(process_timer);