Bridges: use the new findloop for loop detection.

Bridges: use the new findloop for loop detection.

Bridges only needs a loop detector for its non-default 'don't allow
loops' mode. But the one it had was using the graph-pruning strategy,
which means it had the dumb-bell bug - two loops joined by a path
would highlight the path as well as the loops. Switching to the new
findloop system fixes that bug.

A side effect is that I've been able to remove the 'scratch' array
from the game_state, which was only used by the old loop finder, so
that should save memory.