This feature change adds a cross platform sleep and spinlock method with microsecond accuracy.

Change conditionals to require only sys

Reworked the spinlock and usleep implementations and added a timestamp method since we override haxe.Timer and lose access to the high definition stamp for native targets it originally provided.

We need to pass the native time_stamp from cffi for neko and hl.

Merge '8.2.0 dev'

The main thread can easily look these up by ID, and in HTML5, sending the full `JobData` can cause errors.

`__activeThreads` and `__idleThreads` only need to be allocated for multi-threaded pools. Plus, there's no benefit to using a `List` here; we only add to and remove from the end.

And finally, checking `event.job == null` instead of `isOfType()` is faster and avoids an issue in HTML5. Sadly it is less safe, so we might need to revisit it eventually.

It looks like we'll want to take `BackgroundWorker` in a different direction, so for the moment it's safest not to change anything about it. That way, there's only one historical version to maintain backwards compatibility with.

As with `BackgroundWorker`, we're postponing major changes to give us more time to consider.

While I put a lot of effort into that guide, we're changing several things suddenly, and I don't have time to make sure it's up to date.

Restore old `Future` and `BackgroundWorker` behavior.

These were added for drop-in compatibility with `BackgroundWorker`, but we might not need that level of compatibility. We can discuss adding these back later if there's demand.

Hopefully my last before 8.2.0.

Originally had docs for this but we nixed it for a while so they never got added.

Forgot the properties.

We need more docs!

Missing docs.

Offering four options makes the underlying code more complicated for minimal benefit.

merge develop into feature branch