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

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)

test/timer/sleep_float.wren

@@ -1,15 +1,15 @@
 import "timer" for Timer
 
 // These are both rounded to 1, so "a" should complete first.
-Fiber.new {
+Timer.schedule {
   Timer.sleep(1.5)
   IO.print("a")
-}.call()
+}
 
-Fiber.new {
+Timer.schedule {
   Timer.sleep(1.3)
   IO.print("b")
-}.call()
+}
 
 IO.print("main")
 Timer.sleep(3)

test/timer/sleep_return.wren

@@ -0,0 +1,3 @@
+import "timer" for Timer
+
+IO.print(Timer.sleep(0)) // expect: null

test/timer/sleep_zero.wren

@@ -1,27 +1,27 @@
 import "timer" for Timer
 
-Fiber.new {
+Timer.schedule {
+  IO.print("a before")
   Timer.sleep(0)
-  IO.print("a")
-}.call()
+  IO.print("a after")
+}
 
-Fiber.new {
+Timer.schedule {
+  IO.print("b before")
   Timer.sleep(0)
-  IO.print("b")
-}.call()
-
-Fiber.new {
-  Timer.sleep(0)
-  IO.print("c")
-}.call()
+  IO.print("b after")
+}
 
 IO.print("main")
-Timer.sleep(0) // This is enough to let the other fiber run.
+Timer.sleep(0) // This is enough to let the other fibers run to their sleep.
+IO.print("main after")
+Timer.sleep(0) // And now their sleeps complete.
 IO.print("done")
 
 // expect: main
-// Run in the order they were enqueued.
-// expect: a
-// expect: b
-// expect: c
+// expect: a before
+// expect: b before
+// expect: main after
+// expect: a after
+// expect: b after
 // expect: done