Constant babalcore::SLIDING_LENGTH_MAX

pub const SLIDING_LENGTH_MAX: usize = 10_000;

History about one level will be kept only below this limit. Above it, the initial slabs will disappear. This should really not be a problem as for a complete restart, we can reinitialize with a seed, and otherwise it stills allows going back in time by 10k slabs, which means several minutes even at high speed.