doc: mirror image of a DAG is called converse/transpose/reverse graph

It's easier to look for this function if the synonyms are present.
The Wikipedia page refers to the converse / transpose / reverse
terminology.

"mirror image" or "mirror image graph", although quite descriptive,
doesn't always match the existing literature or terminology.

fix documentation and add `shell.nix`

new example: word graph

Fix topological stable sort.

0-1 BFS

Remove the useless dependency on stdlib-shims

ocamlgraph already requires OCaml >= 4.08 which makes stdlib-shims a no-op

Classic: new functions kneser and petersen

Contraction: expose the underlying node mapping

fixed transitive reduction

fixes #145

new example program depend2dot, which turns make-like dependencies
into a DOT graph with transitive reduction

Graphivz: added PosPinned to NeatoAttributes.vertex

closes #143

Oper: improve efficiency of intersect

"Free" improvement of path-check efficiency

Enumerate elementary cycles (Johnson 1975)

two new functions {fold,iter}_component_dist to perform a breadth-first
search from a source, and visit each vertex with the distance from
the source

these were embarassingly not depth-first traversal
see https://11011110.github.io/blog/2013/12/17/stack-based-graph-traversal.html
for an explanation

FASH algorithm for minimal feedback arc sets

Remove the .merlin file