Revert "introduce tainted variables and hashes"

This reverts commit d25832333c195e82cd71eb1d3311ae2dd132d95e.

Only rebuilding tainted nodes wastes too much memory space on some benchmarks.
The problem is that too many nodes may be kept unnecessarily in local tables.

Conflicts:
	library/clp/clpb.pl
ENHANCED: CLP(B): Better support for large conjunctions in sat/1.
ADDED: library(sandbox): current_op/3 is safe.
improve explanation
simplify construction of counter network by removing an impossible case
Updated pengines
rebuild hashes only after all involved BDDs are merged

There is a mistake in the solver (which this commit should fix), found
with the following test case:

        :- use_module(clpb).

        sat(_)  --> [].
        sat(X*Y) --> [_], sat(X), sat(Y).
        sat(X+Y) --> [_], sat(X), sat(Y).
        sat(X#Y) --> [_], sat(X), sat(Y).

        eqs([], _).
        eqs(_, []).
        eqs([V|Vs], [A|As]) :- (   true ; sat(V =:= A) ), eqs(Vs, As).

        run(N) :-
                length(Ls, N),
                portray_clause(N),
                phrase(sat(Sat1), Ls),
                sat(Sat1),
                phrase(sat(Sat2), Ls),
                sat(Sat2),
                term_variables(Sat1, Vs1),
                term_variables(Sat2, Vs2),
                eqs(Vs1, Vs2),
                sat(Sat1 + Sat2),
                false.

        run :- run(_).

Example:

   ?- run.
   0.
   1.
   2.
   3.
   4.
   ERROR: Domain error: `reduced_ites' expected, found `...'

Results with the current commit are still pending.

In any case, tables cannot be cleared prematurely, when the nodes
still occur in some BDDs!
FIXED: Possible crash in dict_ordered()
FIXED: set_breakpoint/4: avoid exception if layout information is only
partially available.
DOC: ~ is not an operator.  Markus Triska.
Updated http
ENHANCED: CLP(B): Improved taut/2 when the second argument is instantiated.
DOC: Updated operator table.  Markus Triska.
Updated semweb
Comment error
ADDED: var_property/2, providing information about a variable for
goal_expansion/2.
Updated pengines
Updated pengines
FIXED: variant_sha1/2 for indirect datatypes (big ints, strings, floats).
Samer Abdallah.
ADDED: library(sandbox): allow using :- op/3 in directives, provided the
declaration is not qualified.