greedy outputs

[mbostock]

Fixes #144 by avoiding pausing the generator in the first place. This changes the semantics such that a generator such as this is greedily evaluated, rather than being ignored unless it is referenced by another code block:

const i = (function* () {
  for (let i = 0; true; ++i) {
    yield i;
  }
})();

I think it’s good that declared generators are greedily evaluated, as it’s probably more expected, and makes it easier to do animation within a cell.