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]

puzzles.h

@@ -234,6 +234,12 @@ void shuffle(void *array, int nelts, int eltsize, random_state *rs);
 void draw_rect_outline(frontend *fe, int x, int y, int w, int h,
                        int colour);
 
+/*
+ * dsf.c
+ */
+int dsf_canonify(int *dsf, int val);
+void dsf_merge(int *dsf, int v1, int v2);
+
 /*
  * version.c
  */