Update to Rust 1.59 and 2021 edition (#3038)

## Proposed Changes

Lots of lint updates related to `flat_map`, `unwrap_or_else` and string patterns. I did a little more creative refactoring in the op pool, but otherwise followed Clippy's suggestions.

## Additional Info

We need this PR to unblock CI.

beacon_node/beacon_chain/src/validator_pubkey_cache.rs

@@ -175,9 +175,7 @@ impl<T: BeaconChainTypes> ValidatorPubkeyCache<T> {
 
     /// Get the `PublicKey` for a validator with `PublicKeyBytes`.
     pub fn get_pubkey_from_pubkey_bytes(&self, pubkey: &PublicKeyBytes) -> Option<&PublicKey> {
-        self.get_index(pubkey)
-            .map(|index| self.get(index))
-            .flatten()
+        self.get_index(pubkey).and_then(|index| self.get(index))
     }
 
     /// Get the public key (in bytes form) for a validator with index `i`.