Symmetric coroutines!

test/fiber/yield_with_value_with_no_caller.wren

@@ -0,0 +1,9 @@
+var a = new Fiber {
+  Fiber.yield(1) // expect runtime error: No fiber to yield to.
+}
+
+// Run a chain of fibers. Since none of them are called, they all get discarded
+// and there is no remaining caller.
+var b = new Fiber { a.run }
+var c = new Fiber { b.run }
+c.run