Add depth-first search algorithms and tests

tests/dfs.rs

@@ -0,0 +1,13 @@
+mod append_graph_tests {
+    use grapherity::models::append_graph::AppendGraph;
+
+    grapherity::graph_topology_test_fixtures!(AppendGraph);
+    grapherity::dfs_tests!(AppendGraph);
+}
+
+mod graph_tests {
+    use grapherity::models::graph::Graph;
+
+    grapherity::graph_topology_test_fixtures!(Graph);
+    grapherity::dfs_tests!(Graph);
+}