Fix Generator yield type

[tetsuo]

Fixes an issue where go can't determine the return type of a yield.

const g = () => {
  return F.go(function*() {
    const thing: number = yield F.resolve(42)
    return F.resolve(43 + thing)
  })
}

(Original bug report on Gitter)

[Avaq]

Thank you Onur! I released this under 12.1.1. :D