Revise low level fiber semantics to play nicer with schedulers.

Now that I'm starting to write a real async scheduler on top of Wren's
basic fiber API, I have a better feel for what it needs. It turns out
run() is not it.

- Remove run() methods.
- Add transfer() which leaves the caller of the invoked fiber alone.
- Add suspend() to return control to the host application.
- Add Timer.schedule() to start a new independently scheduled fiber.
- Change Timer.sleep() so that it only transfers control to explicitly
  scheduled fibers, not any one.

test/core/fiber/call_done.wren

@@ -0,0 +1,6 @@
+var fiber = Fiber.new {
+  IO.print("call")
+}
+
+fiber.call() // expect: call
+fiber.call() // expect runtime error: Cannot call a finished fiber.