Add a `groupBy` goody

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

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

Should we return a Map or an object? Either one could work, though a Map seems more future-looking, since it will support grouping by something other than strings/numbers/Symbols.