Symmetric coroutines!

test/fiber/run_with_value_indirect_reenter.wren

@@ -0,0 +1,20 @@
+var a
+var b
+
+a = new Fiber {
+  IO.print(2)
+  b.run("ignored")
+  IO.print("nope")
+}
+
+b = new Fiber {
+  IO.print(1)
+  a.run("ignored")
+  IO.print(3)
+}
+
+b.call
+// expect: 1
+// expect: 2
+// expect: 3
+IO.print(4) // expect: 4