Add module for Scheduler.

Also reorganizes some code to make it easier to add more modules.

test/scheduler/add.wren

@@ -0,0 +1,15 @@
+import "scheduler" for Scheduler
+import "timer" for Timer
+
+var run = false
+Scheduler.add {
+  run = true
+}
+
+// Does not run immediately.
+IO.print(run) // expect: false
+
+Timer.sleep(0)
+
+// Runs when the main fiber suspends on IO.
+IO.print(run) // expect: true

test/timer/sleep_fibers.wren

@@ -1,11 +1,12 @@
+import "scheduler" for Scheduler
 import "timer" for Timer
 
-Timer.schedule {
+Scheduler.add {
   Timer.sleep(2)
   IO.print("a")
 }
 
-Timer.schedule {
+Scheduler.add {
   Timer.sleep(1)
   IO.print("b")
 }

test/timer/sleep_float.wren

@@ -1,12 +1,13 @@
+import "scheduler" for Scheduler
 import "timer" for Timer
 
 // These are both rounded to 1, so "a" should complete first.
-Timer.schedule {
+Scheduler.add {
   Timer.sleep(1.5)
   IO.print("a")
 }
 
-Timer.schedule {
+Scheduler.add {
   Timer.sleep(1.3)
   IO.print("b")
 }

test/timer/sleep_zero.wren

@@ -1,12 +1,13 @@
+import "scheduler" for Scheduler
 import "timer" for Timer
 
-Timer.schedule {
+Scheduler.add {
   IO.print("a before")
   Timer.sleep(0)
   IO.print("a after")
 }
 
-Timer.schedule {
+Scheduler.add {
   IO.print("b before")
   Timer.sleep(0)
   IO.print("b after")