terraform: refactor Node*Ouput

This commit refactors NodeApplyableOutput and NodeDestroyableOutput into
the new Execute() pattern, collapsing the functions in eval_output.go
into one place.

I also reverted a recent decision to have Execute take a _pointer_ to a
walkOperation: I was thinking of interfaces, not constant bytes, so all
it did was cause problems.

And finally I removed eval_lang.go, which was unused.

terraform/execute.go

@@ -5,5 +5,5 @@ package terraform
 // the process of being removed. A given graph node should _not_ implement both
 // GraphNodeExecutable and GraphNodeEvalable.
 type GraphNodeExecutable interface {
-	Execute(EvalContext, *walkOperation) error
+	Execute(EvalContext, walkOperation) error
 }

terraform/graph_walk_context.go

@@ -167,7 +167,7 @@ func (w *ContextGraphWalker) Execute(ctx EvalContext, n GraphNodeExecutable) tfd
 	// Acquire a lock on the semaphore
 	w.Context.parallelSem.Acquire()
 
-	err := n.Execute(ctx, &w.Operation)
+	err := n.Execute(ctx, w.Operation)
 
 	// Release the semaphore
 	w.Context.parallelSem.Release()

terraform/node_data_destroy.go

@@ -13,7 +13,7 @@ var (
 )
 
 // GraphNodeExecutable
-func (n *NodeDestroyableDataResourceInstance) Execute(ctx EvalContext, op *walkOperation) error {
+func (n *NodeDestroyableDataResourceInstance) Execute(ctx EvalContext, op walkOperation) error {
 	log.Printf("[TRACE] NodeDestroyableDataResourceInstance: removing state object for %s", n.Addr)
 	ctx.State().SetResourceInstanceCurrent(n.Addr, nil, n.ResolvedProvider)
 	return nil

terraform/node_data_destroy_test.go

@@ -36,7 +36,7 @@ func TestNodeDataDestroyExecute(t *testing.T) {
 		}.Instance(addrs.NoKey).Absolute(addrs.RootModuleInstance),
 	}}
 
-	err := node.Execute(ctx, nil)
+	err := node.Execute(ctx, walkApply)
 	if err != nil {
 		t.Fatalf("unexpected error: %s", err.Error())
 	}

terraform/node_data_refresh.go

@@ -191,7 +191,7 @@ type NodeRefreshableDataResourceInstance struct {
 }
 
 // GraphNodeExecutable
-func (n *NodeRefreshableDataResourceInstance) Execute(ctx EvalContext, op *walkOperation) error {
+func (n *NodeRefreshableDataResourceInstance) Execute(ctx EvalContext, op walkOperation) error {
 	addr := n.ResourceInstanceAddr()
 
 	// These variables are the state for the eval sequence below, and are
@@ -280,7 +280,5 @@ func (n *NodeRefreshableDataResourceInstance) Execute(ctx EvalContext, op *walkO
 			return err
 		}
 	}
-
 	return err
-
 }