Make Directory.list() create and return a list from C.

src/vm/wren_debug.c

@@ -20,6 +20,9 @@ void wrenDebugPrintStackTrace(ObjFiber* fiber)
     CallFrame* frame = &fiber->frames[i];
     ObjFn* fn = wrenUpwrapClosure(frame->fn);
 
+    // Skip over stub functions for calling methods from the C API.
+    if (fn->module == NULL) continue;
+    
     // The built-in core module has no name. We explicitly omit it from stack
     // traces since we don't want to highlight to a user the implementation
     // detail of what part of the core module is written in C and what is Wren.

src/vm/wren_vm.c

@@ -1311,23 +1311,30 @@ WrenInterpretResult wrenCall(WrenVM* vm, WrenValue* method)
   ASSERT(vm->fiber->stackTop - vm->fiber->stack >= fn->arity,
          "Stack must have enough arguments for method.");
   
-  callFunction(vm, vm->fiber, (Obj*)fn, fn->arity);
+  // Discard any extra temporary slots. We take for granted that the stub
+  // function has exactly one slot for each arguments.
+  vm->fiber->stackTop = &vm->fiber->stack[fn->maxSlots];
   
+  callFunction(vm, vm->fiber, (Obj*)fn, 0);
   return runInterpreter(vm, vm->fiber);
 }
 
 WrenValue* wrenCaptureValue(WrenVM* vm, Value value)
 {
+  if (IS_OBJ(value)) wrenPushRoot(vm, AS_OBJ(value));
+  
   // Make a handle for it.
   WrenValue* wrappedValue = ALLOCATE(vm, WrenValue);
   wrappedValue->value = value;
 
+  if (IS_OBJ(value)) wrenPopRoot(vm);
+
   // Add it to the front of the linked list of handles.
   if (vm->valueHandles != NULL) vm->valueHandles->prev = wrappedValue;
   wrappedValue->prev = NULL;
   wrappedValue->next = vm->valueHandles;
   vm->valueHandles = wrappedValue;
-
+  
   return wrappedValue;
 }