TarSum: versioning

This introduces Versions for TarSum checksums.
Fixes: https://github.com/docker/docker/issues/7526

It preserves current functionality and abstracts the interface for
future flexibility of hashing algorithms. As a POC, the VersionDev
Tarsum does not include the mtime in the checksum calculation, and would
solve https://github.com/docker/docker/issues/7387 though this is not a
settled Version is subject to change until a version number is assigned.

Signed-off-by: Vincent Batts <vbatts@redhat.com>