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`).

consensus/state_processing/src/consensus_context.rs

@@ -0,0 +1,92 @@
+use std::marker::PhantomData;
+use tree_hash::TreeHash;
+use types::{
+    BeaconState, BeaconStateError, ChainSpec, EthSpec, ExecPayload, Hash256, SignedBeaconBlock,
+    Slot,
+};
+
+#[derive(Debug)]
+pub struct ConsensusContext<T: EthSpec> {
+    /// Slot to act as an identifier/safeguard
+    slot: Slot,
+    /// Proposer index of the block at `slot`.
+    proposer_index: Option<u64>,
+    /// Block root of the block at `slot`.
+    current_block_root: Option<Hash256>,
+    _phantom: PhantomData<T>,
+}
+
+#[derive(Debug, PartialEq, Clone)]
+pub enum ContextError {
+    BeaconState(BeaconStateError),
+    SlotMismatch { slot: Slot, expected: Slot },
+}
+
+impl From<BeaconStateError> for ContextError {
+    fn from(e: BeaconStateError) -> Self {
+        Self::BeaconState(e)
+    }
+}
+
+impl<T: EthSpec> ConsensusContext<T> {
+    pub fn new(slot: Slot) -> Self {
+        Self {
+            slot,
+            proposer_index: None,
+            current_block_root: None,
+            _phantom: PhantomData,
+        }
+    }
+
+    pub fn set_proposer_index(mut self, proposer_index: u64) -> Self {
+        self.proposer_index = Some(proposer_index);
+        self
+    }
+
+    pub fn get_proposer_index(
+        &mut self,
+        state: &BeaconState<T>,
+        spec: &ChainSpec,
+    ) -> Result<u64, ContextError> {
+        self.check_slot(state.slot())?;
+
+        if let Some(proposer_index) = self.proposer_index {
+            return Ok(proposer_index);
+        }
+
+        let proposer_index = state.get_beacon_proposer_index(self.slot, spec)? as u64;
+        self.proposer_index = Some(proposer_index);
+        Ok(proposer_index)
+    }
+
+    pub fn set_current_block_root(mut self, block_root: Hash256) -> Self {
+        self.current_block_root = Some(block_root);
+        self
+    }
+
+    pub fn get_current_block_root<Payload: ExecPayload<T>>(
+        &mut self,
+        block: &SignedBeaconBlock<T, Payload>,
+    ) -> Result<Hash256, ContextError> {
+        self.check_slot(block.slot())?;
+
+        if let Some(current_block_root) = self.current_block_root {
+            return Ok(current_block_root);
+        }
+
+        let current_block_root = block.message().tree_hash_root();
+        self.current_block_root = Some(current_block_root);
+        Ok(current_block_root)
+    }
+
+    fn check_slot(&self, slot: Slot) -> Result<(), ContextError> {
+        if slot == self.slot {
+            Ok(())
+        } else {
+            Err(ContextError::SlotMismatch {
+                slot,
+                expected: self.slot,
+            })
+        }
+    }
+}