Persistence

1. Checkpoint.save writes a sibling .tmp file and replaces the destination. A dump that fails part-way through does not truncate the last good checkpoint.
2. Checkpoint.load restores the constructor's file_path, raise_errors, and make_dir after unpickling, so moving a checkpoint file does not send the next save back to the old path.
3. Setting save_freq = -1 while iterating Checkpoint.range disables further saves. It no longer turns saving on for every remaining generation.