ENHANCED: CLP(B): Earlier release of internal association tables,
                  reducing the library's peak memory consumption.
introduce tainted variables and hashes

We now rebuild tables only when new nodes were created for the
respective variable. This may increase peak memory consumption because
nodes that are no longer needed may still exist in local tables.
rebuild tainted nodes on the fly instead of collecting them first

This reduces the library's peak memory consumption. Always keep in
mind that there may be millions of nodes, and avoid duplicating them.
remove del_attrs/1 call, a (harmless) remain of the old node representation