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/transfer.wren

@@ -0,0 +1,24 @@
+var a = Fiber.new {
+  IO.print("a")
+}
+
+var b = Fiber.new {
+  IO.print("b before")
+  a.transfer()
+  IO.print("b after")
+}
+
+var c = Fiber.new {
+  IO.print("c before")
+  b.transfer()
+  IO.print("c after")
+}
+
+IO.print("start") // expect: start
+
+c.transfer()
+// expect: c before
+// expect: b before
+// expect: a
+
+// Nothing else gets run since the interpreter stops after a completes.