Add push/pop/shift/unshift methods

[assertchris]

No description provided.

[p810]

I've created a pull request for this (#7 ), but I wanted to ask whether you think these methods should mutate the collection, or if they should return a new instance. I'm happy to update them if the latter, I just couldn't tell whether they should be because, while most of the other methods return new instances, their counterparts in the standard lib do as well, e.g. array_filter() to Collection::filter().