Add new benchmarks

Over the last week, the implementation has undergone several improvements.
A notable bug fix is in commit 83198ebe1d4fe807afff39316afde04f910f77e6.
For rigours scientific experiments, we therefore rerun Seed to randomly
generate around 2000 benchmarks consisting of pairs of programs of size 15.
We run these benchmarks with Z3 in less than 24 hours and selected those
that took more than 3 seconds to complete and involving at least one SMT
solver call. The raw benchmark results are included in this patch.