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/fork_revert.rs

@@ -5,7 +5,8 @@ use proto_array::CountUnrealizedFull;
 use slog::{info, warn, Logger};
 use state_processing::state_advance::complete_state_advance;
 use state_processing::{
-    per_block_processing, per_block_processing::BlockSignatureStrategy, VerifyBlockRoot,
+    per_block_processing, per_block_processing::BlockSignatureStrategy, ConsensusContext,
+    VerifyBlockRoot,
 };
 use std::sync::Arc;
 use std::time::Duration;
@@ -172,12 +173,14 @@ pub fn reset_fork_choice_to_finalization<E: EthSpec, Hot: ItemStore<E>, Cold: It
         complete_state_advance(&mut state, None, block.slot(), spec)
             .map_err(|e| format!("State advance failed: {:?}", e))?;
 
+        let mut ctxt = ConsensusContext::new(block.slot())
+            .set_proposer_index(block.message().proposer_index());
         per_block_processing(
             &mut state,
             &block,
-            None,
             BlockSignatureStrategy::NoVerification,
             VerifyBlockRoot::True,
+            &mut ctxt,
             spec,
         )
         .map_err(|e| format!("Error replaying block: {:?}", e))?;