add Util.Graphviz module for printing out graphs for rendering by 'dot'.
add Util.Graphviz module for printing out graphs for rendering by 'dot'.

darcs-hash:20060429234137-1a7c6-a45d778f58e21bbb79241efc539e313e34b7d691

kirbyfan64/jhc,kirbyfan64/jhc,kirbyfan64/jhc,kirbyfan64/jhc