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/rpc/codec/ssz_snappy.rs

@@ -443,7 +443,7 @@ fn handle_length(
         // Note: length-prefix of > 10 bytes(uint64) would be a decoding error
         match uvi_codec.decode(bytes).map_err(RPCError::from)? {
             Some(length) => {
-                *len = Some(length as usize);
+                *len = Some(length);
                 Ok(Some(length))
             }
             None => Ok(None), // need more bytes to decode length