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/beacon_chain.rs

@@ -2289,7 +2289,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                 .position(|(_root, block)| {
                     block.slot().epoch(T::EthSpec::slots_per_epoch()) > start_epoch
                 })
-                .unwrap_or_else(|| filtered_chain_segment.len());
+                .unwrap_or(filtered_chain_segment.len());
 
             // Split off the first section blocks that are all either within the current epoch of
             // the first block. These blocks can all be signature-verified with the same

beacon_node/beacon_chain/src/naive_aggregation_pool.rs

@@ -421,10 +421,7 @@ impl<T: AggregateMap> NaiveAggregationPool<T> {
 
     /// Iterate all items in all slots of `self`.
     pub fn iter(&self) -> impl Iterator<Item = &T::Value> {
-        self.maps
-            .iter()
-            .map(|(_slot, map)| map.get_map().iter().map(|(_key, value)| value))
-            .flatten()
+        self.maps.values().flat_map(|map| map.get_map().values())
     }
 
     /// Removes any items with a slot lower than `current_slot` and bars any future

beacon_node/beacon_chain/src/sync_committee_verification.rs

@@ -635,7 +635,7 @@ pub fn verify_sync_committee_message<T: BeaconChainTypes>(
     let pubkey = pubkey_cache
         .get_pubkey_from_pubkey_bytes(pubkey_bytes)
         .map(Cow::Borrowed)
-        .ok_or_else(|| Error::UnknownValidatorPubkey(*pubkey_bytes))?;
+        .ok_or(Error::UnknownValidatorPubkey(*pubkey_bytes))?;
 
     let next_slot_epoch = (sync_message.get_slot() + 1).epoch(T::EthSpec::slots_per_epoch());
     let fork = chain.spec.fork_at_epoch(next_slot_epoch);

beacon_node/beacon_chain/src/validator_monitor.rs

@@ -198,14 +198,11 @@ impl MonitoredValidator {
     /// as the value recorded by the validator monitor ignores skip slots.
     fn min_inclusion_distance(&self, epoch: &Epoch) -> Option<u64> {
         let summaries = self.summaries.read();
-        summaries
-            .get(epoch)
-            .map(|summary| {
-                summary
-                    .attestation_min_block_inclusion_distance
-                    .map(Into::into)
-            })
-            .flatten()
+        summaries.get(epoch).and_then(|summary| {
+            summary
+                .attestation_min_block_inclusion_distance
+                .map(Into::into)
+        })
     }
 
     /// Maps `func` across the `self.summaries`.

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`.