[jsx-key]: report on `yield`

Seems like yield <Component x={y}/> should be reported by jsx-key if there is no key prop.

This is valid:

for (let [idx, datum] of data.entries()) {
  yield (<MyComponent datum={datum} key={idx} />)
}

This is reported:

for (let [idx, datum] of data.entries()) {
  yield (<MyComponent datum={datum} />) // msg: need `key` prop
}

I can knock this out, if it makes sense.

(cc #332 as it will likely conflict with this if completed concurrently.)