Fibers!

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

test/fiber/yield.wren

@@ -0,0 +1,14 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber 1")
+  Fiber.yield
+  IO.print("fiber 2")
+  Fiber.yield
+  IO.print("fiber 3")
+})
+
+var result = fiber.run // expect: fiber 1
+IO.print("main 1") // expect: main 1
+result = fiber.run // expect: fiber 2
+IO.print("main 2") // expect: main 2
+result = fiber.run // expect: fiber 3
+IO.print("main 3") // expect: main 3