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_transferred.wren

@@ -0,0 +1,12 @@
+var main = Fiber.current
+
+var fiber = Fiber.new {
+  IO.print("transferred")
+  IO.print(main.transfer())
+  IO.print("called")
+}
+
+fiber.transfer() // expect: transferred
+IO.print("main") // expect: main
+fiber.call()     // expect: null
+                 // expect: called