pkg/tarsum: specification on TarSum checksum

Signed-off-by: Vincent Batts <vbatts@redhat.com>
pkg/tarsum: review amendments

(separate commit to preserve github conversation)

Signed-off-by: Vincent Batts <vbatts@redhat.com>
pkg/tarsum: review cleanup

Signed-off-by: Vincent Batts <vbatts@redhat.com>
tarsum: updates for jamtur01 comments

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Merge pull request #9099 from vbatts/vbatts-tarsum_spec

pkg/tarsum: specification on TarSum checksum algorithm