New puzzle: `Slant', picked from the Japanese-language section of

New puzzle: `Slant', picked from the Japanese-language section of
nikoli.co.jp (which has quite a few puzzles that they don't seem to
have bothered to translate into English).

Minor structural change: the disjoint set forest code used in the
Net solver has come in handy again, so I've moved it out into its
own module dsf.c.

[originally from svn r6155]