Retrospective invalidation of exec. payloads for opt. sync (#2837)

## Issue Addressed

NA

## Proposed Changes

Adds the functionality to allow blocks to be validated/invalidated after their import as per the [optimistic sync spec](https://github.com/ethereum/consensus-specs/blob/dev/sync/optimistic.md#how-to-optimistically-import-blocks). This means:

- Updating `ProtoArray` to allow flipping the `execution_status` of ancestors/descendants based on payload validity updates.
- Creating separation between `execution_layer` and the `beacon_chain` by creating a `PayloadStatus` struct.
- Refactoring how the `execution_layer` selects a `PayloadStatus` from the multiple statuses returned from multiple EEs.
- Adding testing framework for optimistic imports.
- Add `ExecutionBlockHash(Hash256)` new-type struct to avoid confusion between *beacon block roots* and *execution payload hashes*.
- Add `merge` to [`FORKS`](https://github.com/sigp/lighthouse/blob/c3a793fd73a3b11b130b82032904d39c952869e4/Makefile#L17) in the `Makefile` to ensure we test the beacon chain with merge settings.
    - Fix some tests here that were failing due to a missing execution layer.

## TODO

- [ ] Balance tests

Co-authored-by: Mark Mackey <mark@sigmaprime.io>

testing/ef_tests/src/cases/fork_choice.rs

@@ -14,15 +14,15 @@ use ssz_derive::Decode;
 use state_processing::state_advance::complete_state_advance;
 use std::time::Duration;
 use types::{
-    Attestation, BeaconBlock, BeaconState, Checkpoint, Epoch, EthSpec, ForkName, Hash256,
-    IndexedAttestation, SignedBeaconBlock, Slot, Uint256,
+    Attestation, BeaconBlock, BeaconState, Checkpoint, Epoch, EthSpec, ExecutionBlockHash,
+    ForkName, Hash256, IndexedAttestation, SignedBeaconBlock, Slot, Uint256,
 };
 
 #[derive(Default, Debug, PartialEq, Clone, Deserialize, Decode)]
 #[serde(deny_unknown_fields)]
 pub struct PowBlock {
-    pub block_hash: Hash256,
-    pub parent_hash: Hash256,
+    pub block_hash: ExecutionBlockHash,
+    pub parent_hash: ExecutionBlockHash,
     pub total_difficulty: Uint256,
 }