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/yield.wren

@@ -6,9 +6,9 @@ var fiber = Fiber.new {
   IO.print("fiber 3")
 }
 
-var result = fiber.call() // expect: fiber 1
+fiber.call()          // expect: fiber 1
 IO.print("main 1")    // expect: main 1
-result = fiber.call() // expect: fiber 2
+fiber.call()          // expect: fiber 2
 IO.print("main 2")    // expect: main 2
-result = fiber.call() // expect: fiber 3
+fiber.call()          // expect: fiber 3
 IO.print("main 3")    // expect: main 3