Add data structures

[domenic]

Closes #4.

[annevk]

This is really great!

• We need to decide whether any of these contains values or elements.
• We their size.
• A sequence should just be a list, no?
• Inside "map", instead of saying "append", should we use "add to the end of the map" since that's the internal terminology we use for lists?

[annevk]

By the way, feel free to merge this once we decide what they contain (values or elements) and use that consistently.

And maybe the map thing. Sequence <> list can be follow-up.

[domenic]

Yeah those make sense.

I'm also wondering if ordered sets should become a type of list, given that that's the plan for stacks/queues as well.

[annevk]

I would be okay with that. I was also wondering if we could have more polymorphous behavior to avoid duplicating all prose.

[domenic]

Yeah. Will fix the easy things and then file follow-up issues and try to address them.