Fix clippy errors

[ackintosh]

Description

This PR fixes the following clippy errors:

• https://rust-lang.github.io/rust-clippy/rust-1.95.0/index.html#iter_kv_map
• https://rust-lang.github.io/rust-clippy/rust-1.95.0/index.html#collapsible_match

It also removes unnecessary mut usages in:

• 3a6f723
• b13cb7c

Note: in b13cb7c, changed PendingNode::value() and PendingEntry::value() were updated to avoid requiring mutable access unnecessarily. Previously, the following code caused a compilation error at service.rs:623:

kbucket::Entry::Pending(ref entry, _) if entry.value().seq() < enr_seq => {
//                                       ^^^^^ cannot borrow immutable local variable `entry` as mutable
    let enr = entry.value().clone();
    to_request_enr = Some(enr);
}

Notes & open questions

Change checklist

• Self-review
• Documentation updates if relevant
• Tests if relevant

[jxs]

LGTM Akihito, we are touching public API's which I feel don't need to be public, let's address that on a future PR before releasing a new minor version