Add BFS/DFS traversal for directed, as well as undirected graphs #517

Job	Run time
Clippy	26s
Build graph-mate with maturin	41s
Rustfmt	22s
Check	1m 16s
Test Suite (stable)	1m 51s
Run python tests	1m 12s
Test Suite (cargo careful)	2m 35s
Test Suite (beta)	1m 59s
Package graph_mate for Mac Universal	0s
Package graph_mate for Mac x86_64	0s
Package graph_mate for Windows	0s
Package graph_mate for Linux	0s
Release all Rust crates	0s
Release a Rust crate	0s
Release graph_mate	0s
	10m 22s

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