[refactor] Refactor Option/Result combinators (#3180)

Code simplifications using `Option`/`Result` combinators to make pattern-matches a tad simpler. 
Opinions on these loosely held, happy to adjust in review.

Tool-aided by [comby-rust](https://github.com/huitseeker/comby-rust).

beacon_node/client/src/config.rs

@@ -149,10 +149,8 @@ impl Config {
     pub fn get_existing_legacy_data_dir(&self) -> Option<PathBuf> {
         dirs::home_dir()
             .map(|home_dir| home_dir.join(&self.data_dir))
-            // Return `None` if the directory does not exists.
-            .filter(|dir| dir.exists())
-            // Return `None` if the legacy directory is identical to the modern.
-            .filter(|dir| *dir != self.get_modern_data_dir())
+            // Return `None` if the legacy directory does not exist or if it is identical to the modern.
+            .filter(|dir| dir.exists() && *dir != self.get_modern_data_dir())
     }
 
     /// Returns the core path for the client.

beacon_node/http_api/tests/tests.rs

@@ -121,8 +121,7 @@ impl ApiTester {
                 harness.chain.slot().unwrap(),
             )
             .into_iter()
-            .map(|vec| vec.into_iter().map(|(attestation, _subnet_id)| attestation))
-            .flatten()
+            .flat_map(|vec| vec.into_iter().map(|(attestation, _subnet_id)| attestation))
             .collect::<Vec<_>>();
 
         assert!(
@@ -244,8 +243,7 @@ impl ApiTester {
                 harness.chain.slot().unwrap(),
             )
             .into_iter()
-            .map(|vec| vec.into_iter().map(|(attestation, _subnet_id)| attestation))
-            .flatten()
+            .flat_map(|vec| vec.into_iter().map(|(attestation, _subnet_id)| attestation))
             .collect::<Vec<_>>();
 
         let attester_slashing = harness.make_attester_slashing(vec![0, 1]);
@@ -2390,8 +2388,7 @@ impl ApiTester {
             .unwrap();
         let attesting_validators: Vec<usize> = committees
             .into_iter()
-            .map(|committee| committee.committee.iter().cloned())
-            .flatten()
+            .flat_map(|committee| committee.committee.iter().cloned())
             .collect();
         // All attesters should now be considered live
         let expected = expected

beacon_node/lighthouse_network/src/discovery/enr.rs

@@ -210,10 +210,8 @@ pub fn load_enr_from_disk(dir: &Path) -> Result<Enr, String> {
     let mut enr_string = String::new();
     match enr_file.read_to_string(&mut enr_string) {
         Err(_) => Err("Could not read ENR from file".to_string()),
-        Ok(_) => match Enr::from_str(&enr_string) {
-            Ok(disk_enr) => Ok(disk_enr),
-            Err(e) => Err(format!("ENR from file could not be decoded: {:?}", e)),
-        },
+        Ok(_) => Enr::from_str(&enr_string)
+            .map_err(|e| format!("ENR from file could not be decoded: {:?}", e)),
     }
 }
 

validator_client/src/duties_service.rs

@@ -646,8 +646,8 @@ async fn poll_beacon_attesters_for_epoch<T: SlotClock + 'static, E: EthSpec>(
         response
             .data
             .into_iter()
-            .filter(|duty| local_pubkeys.contains(&duty.pubkey))
             .filter(|duty| {
+                local_pubkeys.contains(&duty.pubkey) && {
                     // Only update the duties if either is true:
                     //
                     // - There were no known duties for this epoch.
@@ -657,6 +657,7 @@ async fn poll_beacon_attesters_for_epoch<T: SlotClock + 'static, E: EthSpec>(
                             .get(&epoch)
                             .map_or(true, |(prior, _)| *prior != dependent_root)
                     })
+                }
             })
             .collect::<Vec<_>>()
     };

validator_client/src/http_api/tests/keystores.rs

@@ -983,8 +983,7 @@ fn delete_concurrent_with_signing() {
 
         for interchange in collected_slashing_protection
             .into_iter()
-            .map(Result::unwrap)
-            .flatten()
+            .flat_map(Result::unwrap)
         {
             for validator_data in interchange.data {
                 slashing_protection_map