Introduce a new deductive mode in Slant's Hard level, which is the

Introduce a new deductive mode in Slant's Hard level, which is the
generalisation of the previous deduction involving two 3s or two 1s
either adjacent or separated by a row of contiguous 2s. I always
said that was an ugly loop and really ought to arise naturally as a
special case of something more believable, and here it is.

The practical upshot is that Hard mode has just become slightly
harder: some grids generated by the new Slant will be unsolvable by
the old one's solver. I don't think it's become _excessively_ more
hard; I think I'm happy with the new difficulty level. (In
particular, I don't think the new level is sufficiently harder than
the old to make it worth preserving the old one as Medium or
anything like that.)

[originally from svn r6591]