Fibers!

Still have some edge cases to implement but the basics are in
place and working now.

test/fiber/resume_caller.wren

@@ -0,0 +1,19 @@
+var b = Fiber.create(fn {
+  IO.print("fiber b")
+})
+
+var a = Fiber.create(fn {
+  IO.print("begin fiber a")
+  b.run
+  IO.print("end fiber a")
+})
+
+IO.print("begin main")
+a.run
+IO.print("end main")
+
+// expect: begin main
+// expect: begin fiber a
+// expect: fiber b
+// expect: end fiber a
+// expect: end main