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/timer/sleep_fibers.wren

@@ -1,14 +1,14 @@
 import "timer" for Timer
 
-Fiber.new {
+Timer.schedule {
   Timer.sleep(2)
   IO.print("a")
-}.call()
+}
 
-Fiber.new {
+Timer.schedule {
   Timer.sleep(1)
   IO.print("b")
-}.call()
+}
 
 IO.print("main")
 Timer.sleep(3)