Add setCurrent() method to Page Collection

[Karmalakas]

Lets us have a workaround for #531
Also was a discussion on Discourse

This new method would allow to have expected (not inversed) prev and next items of the collection in Twig
Eg.:

    {% set collection = page.parent.collection({items: '@self.children', filter: {published: true, routable: true}}) %}
    {% set isFirst = collection.isFirst(page.path) %}
    {% set isLast = collection.isLast(page.path) %}

    <section>
        {% if not isFirst %}
            {% do collection.setCurrent(page.path) or collection.prev %}
            <a href="{{ collection.current.url }}">Previous page</a>
        {% endif %}

        {% if not isLast %}
            {% do collection.setCurrent(page.path) or collection.next %}
            <a href="{{ collection.current.url }}">Next page</a>
        {% endif %}
    </section>

[mahagr]

This is not a bad idea as it's not limited to siblings but next/previous items in the whole collection, which can contain any page on the site.

However, there's minor room for improvement.

[mahagr]

I would get the key into a variable to speed up the loop.

[Karmalakas]

But key setting will have to be inside the loop anyway 🤔 Will see what I can come up with. Also will check how expensive this is (I know it might be 2x faster maybe, but really doubt there will be a significant difference)

[Karmalakas]

Is it worth introducing a variable to gain 250ms in a 1M items collection? 🙂 Ran a few times and one time result was opposite even 🙂

[mahagr]

It's 9% faster. :) Generally I prefer well-written code over repeating the same thing over and over again.

while (($key = key($this->items)) !== null && $key !== $path) {
    next($this->items);
}

[Karmalakas]

Ping @mahagr 🙂
Not sure if re-request of review is enough or should I ping next time right after re-request 😄