lang: funcs, interfaces: Add the Graph method

The Txn wants to be able to call graph directly.

lang/funcs/txn/txn.go

@@ -624,3 +624,12 @@ func (obj *GraphTxn) Free() {
 		obj.FreeFunc()
 	}
 }
+
+// Graph returns a copy of the contained graph. It returns what has been already
+// committed.
+func (obj *GraphTxn) Graph() *pgraph.Graph {
+	obj.mutex.Lock()
+	defer obj.mutex.Unlock()
+
+	return obj.GraphAPI.Graph() // returns a copy
+}

lang/interfaces/func.go

@@ -339,4 +339,8 @@ type Txn interface {
 	// separate state. This allows you to do an Add*/Commit/Reverse that
 	// isn't affected by a different user of this transaction.
 	Copy() Txn
+
+	// Graph returns a copy of the graph. It returns what has been already
+	// committed.
+	Graph() *pgraph.Graph
 }