refactor: ♻️ more functional Programming

[KP64]

This PR only serves the purpose of cleaning up the Code a bit, be it in form of formatting more functional approaches or compressing functions (map().filter() → filter_map()).

I implemented Add & AddAssign traits for FileCount. This allows for a more functional approach, when calculating the file count, for example.

This:

pub fn compute_file_count(node_id: NodeId, tree: &Arena<Node>) -> FileCount {
    let mut count = FileCount::default();
    for child_id in node_id.children(tree) {
        count.update(tree[child_id].get());
    }
    count
}

became this:

pub fn compute_file_count(node_id: NodeId, tree: &Arena<Node>) -> FileCount {
    node_id
        .children(tree)
        .map(|child_id| tree[child_id].get())
        .fold(FileCount::default(), |acc, node| acc + node)
}

The changes were Tested on WSL (Ubuntu) and it was only checked whether it compiles on Windows (refer to #180 as to why it was not tested on Windows).

[solidiquis]

lgtm