[ccan] RFC: Abstract Graph Algorithms

[David Gibson]

This patch series implements two new modules, implementing some
standard graph algorithms.  It's just depth and breadth first search,
for now, but I plan to add Dijkstra's algorithm.

These are "abstract" in the sense that they don't require a specific
graph representation, instead allowing the caller to provide callbacks
which will compute the edges, and even construct nodes on the fly from
some other representation.

Caveats - suggestions to improve these more than welcome

  * I don't love the name

  * The void * passed to the edge function to let it store internal
    state (e.g. a index / loop counter) is pretty clumsy

  * Various things could probably be improved with typesafe_cb

  * Needs more examples and tests, in particular ones which actually
    construct nodes on the fly.