SF/173: place a limit on non-deterministic expansion

[pabigot]

SF/173

Schema that contain the equivalent of (a*)* (as in trac/172) are non-deterministic, and the number of pending unresolved automaton configurations will grow exponentially until a symbol match eliminates some of them. Impose a configurable limit on the amount of non-determinism, and allow the user to control whether exceeding that limit produces an error or simply resolves it arbitrarily.