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`).

lcli/src/skip_slots.rs

@@ -121,7 +121,7 @@ pub fn run<T: EthSpec>(mut env: Environment<T>, matches: &ArgMatches) -> Result<
     };
 
     for i in 0..runs {
-        let mut state = state.clone_with(CloneConfig::committee_caches_only());
+        let mut state = state.clone_with(CloneConfig::all());
 
         let start = Instant::now();
 

lcli/src/transition_blocks.rs

@@ -74,7 +74,7 @@ use eth2::{
 use ssz::Encode;
 use state_processing::{
     block_signature_verifier::BlockSignatureVerifier, per_block_processing, per_slot_processing,
-    BlockSignatureStrategy, VerifyBlockRoot,
+    BlockSignatureStrategy, ConsensusContext, VerifyBlockRoot,
 };
 use std::borrow::Cow;
 use std::fs::File;
@@ -360,6 +360,7 @@ fn do_transition<T: EthSpec>(
             decompressor,
             &block,
             Some(block_root),
+            Some(block.message().proposer_index()),
             spec,
         )
         .map_err(|e| format!("Invalid block signature: {:?}", e))?;
@@ -367,12 +368,15 @@ fn do_transition<T: EthSpec>(
     }
 
     let t = Instant::now();
+    let mut ctxt = ConsensusContext::new(pre_state.slot())
+        .set_current_block_root(block_root)
+        .set_proposer_index(block.message().proposer_index());
     per_block_processing(
         &mut pre_state,
         &block,
-        None,
         BlockSignatureStrategy::NoVerification,
         VerifyBlockRoot::True,
+        &mut ctxt,
         spec,
     )
     .map_err(|e| format!("State transition failed: {:?}", e))?;