Fibers!

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

test/fiber/run.wren

@@ -0,0 +1,12 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+})
+
+IO.print("before") // expect: before
+fiber.run          // expect: fiber
+IO.print("after")  // expect: after
+
+// TODO: Test handles error if fiber tries to run itself.
+// TODO: Test create is passed right argument type.
+// TODO: Test closing over stuff in fiber function.
+// TODO: Test running a finished fiber.