Provide a relation oracle for paths.

This provides a path_oracle class which can optionally be used in conjunction
with another oracle to track relations on a path as it is walked.

	* value-relation.cc (class equiv_chain): Move to header file.
	(path_oracle::path_oracle): New.
	(path_oracle::~path_oracle): New.
	(path_oracle::register_relation): New.
	(path_oracle::query_relation): New.
	(path_oracle::reset_path): New.
	(path_oracle::dump): New.
	* value-relation.h (class equiv_chain): Move to here.
	(class path_oracle): New.