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.

include/types.h

@@ -37,6 +37,7 @@
 typedef unsigned char natid;	/* NSC_NATID must match this */
 typedef short coord;
 
+struct bp;
 struct emp_qelem;
 struct empobj;
 struct lndstr;