Add Arena::with_capacity

[slivering]

When creating large trees, inserting many nodes causes the underlying Vec to reallocate frequently.
Arena::with_capacity would simply create allocate the Vec beforehand.

fn with_capacity(capacity: usize) -> Self {
    Self {
        nodes: Vec::with_capacity(capacity),
        first_free_slot: None,
        last_free_slot: None,
    }
}

I would also suggest adding the Arena::clear function which simply clears the Vec, causing all the created NodeIds to be invalid.
This is equivalent to removing all the Nodes while retaining the Vec capacity.
Unless I forgot something, I'm rather confident that such an implementation would not cause soundness issues, e.g. with the iterators.

[saschagrunert]

@slivering this sounds like a reasonable enhancement, do you plan to bring up a PR for that?