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>

lcli/src/create_payload_header.rs

@@ -23,7 +23,7 @@ pub fn run<T: EthSpec>(matches: &ArgMatches) -> Result<(), String> {
         base_fee_per_gas,
         timestamp: genesis_time,
         block_hash: eth1_block_hash,
-        random: eth1_block_hash,
+        random: eth1_block_hash.into_root(),
         ..ExecutionPayloadHeader::default()
     };
     let mut file = File::create(file_name).map_err(|_| "Unable to create file".to_string())?;

lcli/src/new_testnet.rs

@@ -108,7 +108,7 @@ pub fn run<T: EthSpec>(testnet_dir_path: PathBuf, matches: &ArgMatches) -> Resul
         let genesis_state = interop_genesis_state::<T>(
             &keypairs,
             genesis_time,
-            eth1_block_hash,
+            eth1_block_hash.into_root(),
             execution_payload_header,
             &spec,
         )?;