Replace the revolting build pointer data structure by a proper data

Replace the revolting build pointer data structure by a proper data
type.  Make it abstract because that's possible.  Change data layout
so that the slots belonging to a sector are together in memory, it's
nicer to the cache.
(bp): The new type.  Users changed.
(get_wp): Update accordingly.
(alloc_bp): New.
(update_main, calc_all): Use it.  Before, calc_all() allocated 1/7
more than necessary.