Improve the Flood solver.

Previously it simply chose every move based on the static evaluation
function 'minimise the pair (longest shortest-path to any square,
number of squares at that distance)'. Now it looks three moves ahead
recursively, and I've also adjusted the evaluation function to tie-
break based on the number of squares brought to distance zero (i.e.
actually in control).

The result isn't an unconditional improvement on the old solver; in a
test run based on 'flood --generate 1000 12x12c6m0#12345' I found that
57 out of 1000 grids tested now had longer solutions. However, about
three quarters had shorter ones, and solutions are more than a move
shorter on average.

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