Add Ganeti.HTools.Graph

This module implements some algorithms on Data.Graph data structures.
At the moment its main functionality is an LF-color implementation
(greedy coloring in descending order of degree). There are also a few
extra functions to calculate the degree order, and convert the node to
color mapping to color to nodes.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>