- Delete unused vy_page_info->min_key_offset.
 - Move vy_run_info->{size,keys,page_infos} to vy_run, because
   they are not a part of run info stored in .index file.
 - Rename vy_run->page_infos to page_info.
 - Rename vy_page_info->count, vy_run->keys, and vy_slice->keys
   to row_count.
 - Update comments.