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_called_fibers.wren

@@ -0,0 +1,21 @@
+import "timer" for Timer
+
+var a = Fiber.new {
+  IO.print("a before")
+  Timer.sleep(10)
+  IO.print("a after")
+}
+
+var b = Fiber.new {
+  IO.print("b before")
+  a.call()
+  IO.print("b after")
+}
+
+// All fibers are suspended since they were directly called and not scheduled.
+IO.print("before")  // expect: before
+b.call()            // expect: b before
+                    // expect: a before
+                    // expect: a after
+                    // expect: b after
+IO.print("done")    // expect: done