Add Fiber.try().

test/fiber/try_indirect_reenter.wren

@@ -0,0 +1,12 @@
+var a
+var b
+
+a = new Fiber {
+  b.try // expect runtime error: Fiber has already been called.
+}
+
+b = new Fiber {
+  a.call
+}
+
+b.call