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.

script/test.py

@@ -149,7 +149,7 @@ class Test:
 
   def validate_runtime_error(self, error_lines):
     if len(error_lines) < 2:
-      self.fail('Expected runtime error "{0} and got none.',
+      self.fail('Expected runtime error "{0}" and got none.',
           self.runtime_error_message)
       return