Compute Hash of Node

Summary:
Add method and algorithm to compute the Graph hash:

 * if a Node is a leaf: hash = hash(content of the file)
 * if a Node is not a leaf: hash = hash('node name' .  hash(childRule))
 * For a rule: hash = hash('rule cmd' . hash(inputNode1) . hash(inputNode2)...)

In order to be able to check everything is alright, the module Graphiz will use
the hash instead of the Node/rule address as identifier.

If there is a mess or a loop, then there is errors

Test Plan: use -M dot

Reviewers: adrienconrath

Reviewed By: adrienconrath

Subscribers: NicolasDP

Differential Revision: http://91.121.199.76/D43