Add a tally goody #109
Description
Inspiration from Ruby: https://rubydoc.info/stdlib/core/Enumerable:tally
And Python: https://docs.python.org/3/library/collections.html#collections.Counter
Going with the Ruby name since it's a verb.
A reasonable place for it to live would be on Iterator.prototype, then anything that can give us an iterator can also be tallied.
Should we return a Map or an object? Either one could work, though a Map seems more future-looking, since it will support tallying something other than strings/numbers/Symbols.