Add an `export` builtin, and API to write to libarchive

Add an `export` builtin, and API to write to libarchive

At the moment I'm looking at using rpm-ostree to manage RPM inputs
which can then be converted into Docker images.  It's most convenient
if we can stream directly out of libostree rather than doing a
checkout + tar combination.

There are also backup/debugging etc. reasons to implement `export` as
well.