Add Graph::withoutVertex() and Graph::withoutEdge() instead of destroy()

[clue]

Destroying a Vertex or Edge will no longer leave an object in an invalid
state but will instead return a new graph without the given entity. Also
add Graph::withoutVertices() and Graph::withoutEdges() to remove
multiple entities at once.

Accordingly, Walk::isValid() has been removed as it would always return
true.

// old
$vertex->destroy();
$edges->destroy();

// new
$graph = $graph->withoutVertex($vertex);
$graph = $graph->withoutEdge($edge);

Builds on top of #188