Clippy lints for rust 1.66 (#3810)

## Issue Addressed
Fixes the new clippy lints for rust 1.66

## Proposed Changes

Most of the changes come from:
- [unnecessary_cast](https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast)
- [iter_kv_map](https://rust-lang.github.io/rust-clippy/master/index.html#iter_kv_map)
- [needless_borrow](https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow)

## Additional Info

na

beacon_node/lighthouse_network/src/service/utils.rs

@@ -88,7 +88,7 @@ fn keypair_from_hex(hex_bytes: &str) -> error::Result<Keypair> {
         hex_bytes.to_string()
     };
 
-    hex::decode(&hex_bytes)
+    hex::decode(hex_bytes)
         .map_err(|e| format!("Failed to parse p2p secret key bytes: {:?}", e).into())
         .and_then(keypair_from_bytes)
 }