Reimplement RDFS module using sound backward chaining

[wouterbeek]

Made a first attempt that is not very performant. There seems to be some looping.

Proposed API:

• rdf_mt(?S, ?P, ?O)
• rdf_mt(?S, ?P, ?O, -Tree)
• rdf_property(?P)
• rdfs_class(?C)
• rdfs_instance(?I, ?C)
• rdfs_property(?P)
• rdfs_subclass(?C, ?D)
• rdfs_subproperty(?P, ?Q)

Improvements WRT rdfs.pl:

• rdf_mt/3 allows all entailment results to be generated, not only sublclass, subproperty and instanace relations. (The class/property/instance predicates are all implemented as simple wrappers of rdf_mt/3.)
• rdf_mt/4 gives a proof tree for each match.
• Rename "individual" -> "instance"
• rdfs:domain and rdfs:range are not used by the instance-of relation.
• Interactions between rdfs:subPropertyOf are other entailment rules are not supported.

Two possible solutions for the looping:

• Tabling
• TMS