tarsum: adding benchmarks

to cover a couple of use-cases:
* 1mb file, using no compression
* 1mb file, using compression
* 1024 1k files, using no compression
* 1024 1k files, using compression

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)