Package goroutine provides a single function that will return the runtime's ID number for the calling goroutine.
The implementation is derived from Laevus Dexter's comment in Gophers' Slack #darkarts, https://gophers.slack.com/archives/C1C1YSQBT/p1593885226448300 post which linked to this playground snippet https://play.golang.org/p/CSOp9wyzydP.
The code here is an exercise in minimalism, doing as little as possible by
deferring nearly all of the logic to runtime functions co-opted via
//go:linkname comments.