Convert Tents to use matching instead of maxflow.

Convert Tents to use matching instead of maxflow.

Tents needs to construct maximal matchings in two different
situations. One is the completion check during play, in which the
existence of a perfect matching between tents and trees is part of the
win condition; the other is the initial grid generation, in which we
find a _maximal_ matching between the tents we've already placed and
all the possible neighbouring squares that are candidates for the tree
positions. Both of those are switched over.