Remove Vertices and Edges collection classes, use plain arrays instead

[clue]

This changeset removes the Vertices and Edges collection classes and changes the code to use plain arrays instead. These collection classes (incorrectly named "sets") added some very significant complexity to the code base and bring little value for this package. Some of their methods were specifically designed for specific algorithms, so we'll port these over to graphp/algorithms as appropriate (#119).

// old
$vertices = $edge->getVertices()->getVector();
$edges = $vertex->getEdges()->getVector();
$vertex = $edge->getVertices()->getVertexFirst();
$edge = $vertex->getEdges()->getEdgeFirst();

// new
$vertices = $edge->getVertices();
$edges = $vertex->getEdges();
$vertex = \reset($vertices);
$edge = \reset($edges);

Builds on top of #188, #185 and others.