[BUG] ProjectedToSimple vs. Flatten: could they be unified?

**Describe the bug**

Current `project_to_simple` is bad in both performance and memory usage, as it will copy the projected v/e's structure, even when the graph has only one vertex label and one edge label.

That is really bad, as the cost of `project` will greater than the cost of running an application.

Now we have flatten fragment, could the `ProjectedFragment` be replaced with the flatten fragment? Or, could we make it doing less copy? e.g., even a special case for graphs that contains only one vertex label and one edge label.

**Additional context**

Related to #1300.