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.

src/module/timer.c

@@ -14,16 +14,36 @@ static const char* timerLibSource =
 "    if (!(milliseconds is Num)) Fiber.abort(\"Milliseconds must be a number.\")\n"
 "    if (milliseconds < 0) Fiber.abort(\"Milliseconds cannot be negative.\")\n"
 "    startTimer_(milliseconds, Fiber.current)\n"
-"    Fiber.yield()\n"
+"\n"
+"    runNextScheduled_()\n"
+"  }\n"
+"\n"
+"  // TODO: Once the CLI modules are more fleshed out, find a better place to\n"
+"  // put this.\n"
+"  static schedule(callable) {\n"
+"    if (__scheduled == null) __scheduled = []\n"
+"    __scheduled.add(Fiber.new {\n"
+"      callable.call()\n"
+"      runNextScheduled_()\n"
+"    })\n"
 "  }\n"
 "\n"
 "  foreign static startTimer_(milliseconds, fiber)\n"
 "\n"
 "  // Called by native code.\n"
 "  static resumeTimer_(fiber) {\n"
-"    fiber.run()\n"
+"    fiber.transfer()\n"
 "  }\n"
-"}\n";
+"\n"
+"  static runNextScheduled_() {\n"
+"    if (__scheduled == null || __scheduled.isEmpty) {\n"
+"      Fiber.suspend()\n"
+"    } else {\n"
+"      __scheduled.removeAt(0).transfer()\n"
+"    }\n"
+"  }\n"
+"}\n"
+"\n";
 
 // The Wren method to call when a timer has completed.
 static WrenMethod* resumeTimer;

src/module/timer.wren

@@ -3,13 +3,33 @@ class Timer {
     if (!(milliseconds is Num)) Fiber.abort("Milliseconds must be a number.")
     if (milliseconds < 0) Fiber.abort("Milliseconds cannot be negative.")
     startTimer_(milliseconds, Fiber.current)
-    Fiber.yield()
+
+    runNextScheduled_()
+  }
+
+  // TODO: Once the CLI modules are more fleshed out, find a better place to
+  // put this.
+  static schedule(callable) {
+    if (__scheduled == null) __scheduled = []
+    __scheduled.add(Fiber.new {
+      callable.call()
+      runNextScheduled_()
+    })
   }
 
   foreign static startTimer_(milliseconds, fiber)
 
   // Called by native code.
   static resumeTimer_(fiber) {
-    fiber.run()
+    fiber.transfer()
+  }
+
+  static runNextScheduled_() {
+    if (__scheduled == null || __scheduled.isEmpty) {
+      Fiber.suspend()
+    } else {
+      __scheduled.removeAt(0).transfer()
+    }
   }
 }
+