Add a new assembler that takes a tree and creates a Open Container
Initiative[2] image according to the OCI image format[2]. The final
result is a tarball, aka a "orci-archive", that can be pulled into
podman with `podman pull oci-archive:<archive>`. Currently the only
required options are `filename` and `architecture`.
[1] https://www.opencontainers.org/
[2 ]https://github.com/opencontainers/image-spec/
