Fibers!

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

test/fiber/run_with_value.wren

@@ -0,0 +1,9 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+})
+
+// The first value passed to the fiber is ignored, since there's no yield call
+// to return it.
+IO.print("before")   // expect: before
+fiber.run("ignored") // expect: fiber
+IO.print("after")    // expect: after