Fix Rust 1.61 clippy lints (#3192)

## Issue Addressed

This fixes the low-hanging Clippy lints introduced in Rust 1.61 (due any hour now). It _ignores_ one lint, because fixing it requires a structural refactor of the validator client that needs to be done delicately. I've started on that refactor and will create another PR that can be reviewed in more depth in the coming days. I think we should merge this PR in the meantime to unblock CI.

beacon_node/network/src/beacon_processor/mod.rs

@@ -42,6 +42,7 @@ use crate::sync::manager::BlockProcessType;
 use crate::{metrics, service::NetworkMessage, sync::SyncMessage};
 use beacon_chain::parking_lot::Mutex;
 use beacon_chain::{BeaconChain, BeaconChainTypes, GossipVerifiedBlock};
+use derivative::Derivative;
 use futures::stream::{Stream, StreamExt};
 use futures::task::Poll;
 use lighthouse_network::{
@@ -51,7 +52,6 @@ use lighthouse_network::{
 use logging::TimeLatch;
 use slog::{crit, debug, error, trace, warn, Logger};
 use std::collections::VecDeque;
-use std::fmt;
 use std::pin::Pin;
 use std::sync::{Arc, Weak};
 use std::task::Context;
@@ -331,17 +331,13 @@ impl DuplicateCache {
 }
 
 /// An event to be processed by the manager task.
+#[derive(Derivative)]
+#[derivative(Debug(bound = "T: BeaconChainTypes"))]
 pub struct WorkEvent<T: BeaconChainTypes> {
     drop_during_sync: bool,
     work: Work<T>,
 }
 
-impl<T: BeaconChainTypes> fmt::Debug for WorkEvent<T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{:?}", self)
-    }
-}
-
 impl<T: BeaconChainTypes> WorkEvent<T> {
     /// Create a new `Work` event for some unaggregated attestation.
     pub fn unaggregated_attestation(
@@ -615,7 +611,8 @@ impl<T: BeaconChainTypes> std::convert::From<ReadyWork<T>> for WorkEvent<T> {
 }
 
 /// A consensus message (or multiple) from the network that requires processing.
-#[derive(Debug)]
+#[derive(Derivative)]
+#[derivative(Debug(bound = "T: BeaconChainTypes"))]
 pub enum Work<T: BeaconChainTypes> {
     GossipAttestation {
         message_id: MessageId,

beacon_node/network/src/sync/range_sync/chain.rs

@@ -26,8 +26,9 @@ const BATCH_BUFFER_SIZE: u8 = 5;
 /// A return type for functions that act on a `Chain` which informs the caller whether the chain
 /// has been completed and should be removed or to be kept if further processing is
 /// required.
-#[must_use = "Should be checked, since a failed chain must be removed. A chain that requested
- being removed and continued is now in an inconsistent state"]
+///
+/// Should be checked, since a failed chain must be removed. A chain that requested being removed
+/// and continued is now in an inconsistent state.
 pub type ProcessingResult = Result<KeepChain, RemoveChain>;
 
 /// Reasons for removing a chain