[Do not merge, RFC]Add dataflow!

With futures, you can call ```value!``` if you want exceptions to bubble up. It'd be rather nice if you could do the same thing with dataflow.

This is the simplest possible solution; if you collectively think this is a worthwhile feature I'll polish it up, add tests, and so forth. Suggestions on implementation welcome.
This turned out to be duplicated in the spec, so I removed a copy.