core.thread: New thread_{get,set}Suspendable() API.

This is useful for high-priority threads that absolutely must not be suspended by e.g. the garbage collector. One could achieve the same effect by detaching the thread from the runtime, but that's a rather roundabout way to do it, since most runtime functions will then not work.

As the docs imply, this is incredibly unsafe and has many gotchas, so it should be used with extreme care.