Remove fake nanoseconds

[ptomato]

Follow-up from #162 (comment)

Currently the "nanoseconds" in Temporal.Now.* are faked, since the current time is obtained from Date.now(). This was carried over from the Temporal proposal's reference polyfill. Since this polyfill is intended for production, we should remove the fake nanoseconds. There's probably some rollup transform that will give us process.hrtime in Node and a lower-precision equivalent in the browser.