Tracks: new neighbour-based deduction.

Tracks: new neighbour-based deduction.

This is a deduction I've been using in my own head for years: if you
only have one remaining filled square to put in a row, then it can't
be any square that has two adjacent edges blocked, because if that
square contains anything at all then it would have to be a corner
piece, and a corner piece forces the square next to it to be filled as
well.

I ran across a puzzle today that this implementation couldn't solve,
but I solved it fine by hand and found the deduction I was using that
wasn't implemented here. Now it is.