complexity: use `extend` instead of `append`

What it does

Collections (BinaryHeap, BTreeMap, BTreeSet, LinkedList, VecDeque, Vec) can be grown with both append and extend:

things.append(&mut more_things);
things.extend(more_things);

This can be applied in case more_things isn't re-used later (extend consumes more_things), things has an impl Extend and more_things has an impl IntoIterator.

Advantage

• less cognitive complexity, because you don't need the &mut borrow
• it's shorter 😅

Drawbacks

I don't know if there is overhead / a performance impact of using extend instead of append.

Example

things.append(&mut more_things);

Could be written as:

things.extend(more_things);

Start from the Rust examples in the issue and identify the Clippy lint entry point and relevant tests; no file or test paths are provided. Establish whether replacing append with extend is safe under the stated ownership and trait conditions, including any performance impact, and add coverage showing the intended cases are reported.