1.0.2.12: New hash-based implementation of ssets

1.0.2.12: New hash-based implementation of ssets

        * The old version that used sorted lists had bad worst case performance,
          which was especially noticeable with constraint propagation on
          hairy functions.
        * Use yet another custom hash implementation (with open addressing
          and double hashing), since the standard hash-tables are too heavy
          for this (e.g. locking overhead, memory consumption).
        * An sset implementation based on balanced trees was also tested,
          but in practice turned out to be even slower than the sorted lists,
          due to the high
        * DO-SSET-ELEMENTS no longer iterates in SSET-ELEMENT-NUMBER order,
          but we don't seem to rely on the old behaviour anywhere.