1.0.12.18: faster member-type operations

1.0.12.18: faster member-type operations

* XSET is a generic set implementation, that uses lists of small sets,
  and switches to hashes for larger ones. Current switchoff point is
  12 -- but some operations would benefit from a larger one. TODO:
  There are other places in SBCL that will probably want to use XSET
  as well.

* Instead of storing members directly in the set object, store them in
  an XSET -- except for floating point zeros which go into a list of
  their own, simplifying the canonicalization a bit. (By adding
  complexity elsewhere, of course. Maybe this is not TRT after all...)

* ...now member type arithmetic is mostly O(1) or O(N), instead of
  O(BAD), but some operations cons more then before: old implemenation
  manageg eg. union without consing when either set was the subset of
  the other one -- not so anymore.