#865 Use relnext and relprev for improving data based synthesis performance

Closes #865 (closed).

Results of running the CIF benchmark: benchmark_results.zip. The HTML report of the CIF benchmark results contains two configurations. Config 1 is the baseline, for which the develop branch (commit 1fbb8818) was used. Config 2 are the changes proposed in this PR, i.e., relprev and relnext. On average, peak memory consumption (the max number of used BDD nodes) is slightly better (and will further improve as result of #866 (closed)), and performance (the average number of BDD operations / cache misses) has improved by almost a factor 3.