Add a `partition` goody

Inspiration from Ruby: https://rubydoc.info/stdlib/core/Enumerable:partition

A reasonable place for it to live would be on `Iterator.prototype`, then anything that can give us an iterator can also be partitioned.

Should we return two arrays like Ruby does or two iterators? I'm currently thinking that two arrays is fine. Two iterators would only be better if these iterators are lazy, but that doesn't seem worth the overhead currently.