To do that, I've reorganized the code

Test: with ocaml 4.08, it is impossible to load the same function twice.
Hence the need to define p.mli files for graphs with several nodes.

Also, track missing files and the alea-coloring exemples.

For some reason, I believed that the source dir containing the main
program need to be named bin for dune to work smoothly. But actually,
it seems to work fine with src, so let's go for it.