Fix mismatched_lifetime_syntaxes warnings

[MonterraByte]

This PR fixes a bunch of warnings like the following one:

warning: hiding a lifetime that's elided elsewhere is confusing
  --> src/node/node_mut.rs:17:46
   |
17 |     pub(crate) fn new(node_id: NodeId, tree: &mut Tree<T>) -> NodeMut<T> {
   |                                              ^^^^^^^^^^^^     ^^^^^^^^^^ the same lifetime is hidden here
   |                                              |
   |                                              the lifetime is elided here
   |
   = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
   = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: use `'_` for type paths
   |
17 |     pub(crate) fn new(node_id: NodeId, tree: &mut Tree<T>) -> NodeMut<'_, T> {
   |