Enhance Filling's solver to handle large ghost regions.

The previous solver could cope with inferring a '1' in an empty
square, but had no deductions that would enable it to infer the
existence of a '4'-sized region in 5x3:52d5b1a5b3. The new solver can
handle that, and I've made a companion change to the clue-stripping
code so that it aims to erase whole regions where possible so as to
actually present this situation to the player.

Current testing suggests that at the smallest preset a nontrivial
ghost region comes up in about 1/3 of games, and at the largest, more
like 1/2 of games. I may yet decide to introduce a difficulty level at
which it's skewed to happen more often still and one at which it
doesn't happen at all; but for the moment, this at least gets the
basic functionality into the code.

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