Add 'stream' and 'future' types

[lukewagner]

This PR builds on #363 to add future and stream type constructors along with the associated canon built-ins ({stream,future}.new, {stream,future}.{read,write}, {stream,future}.cancel-{read,write}). It also generalizes the "async_subtasks" table to also hold futures and streams, renaming it to be the "waitables" table and renaming subtask.drop to waitable.drop. Lastly, a new canonopt called always-task-return is added so that sync-lifted exports can use task.return to return their value, which you need if you want to return a stream or future from a synchronous function.

I'd suggest reading the new text in Explainer.md/Binary to get a short overview of the concrete syntactic/binary additions, then Async.md to get the high-level summary, then CanonicalABI.md to get the full details.