Foundation for native CLP(FD). The substitution dict carries a reserved
"_fd" key holding a constraint store:
{:domains {var-name -> sorted-int-list}
:constraints (... pending constraints ...)}
This commit ships only the domain machinery + accessors:
fd-dom-from-list / fd-dom-range / fd-dom-empty? / fd-dom-singleton?
fd-dom-min / fd-dom-max / fd-dom-member? / fd-dom-intersect /
fd-dom-without
fd-store-of / fd-domain-of / fd-set-domain / fd-with-store
fd-set-domain returns nil when the domain becomes empty (failure),
which is the wire signal subsequent constraint goals will consume.
The constraints field is reserved for the next iteration.
26 new tests, 577/577 cumulative.
