*pack: implement rootless unpacking/repacking

[cyphar]

Currently umoci unpack requires root privileges to set the owners of the files, and thus umoci repack sometimes needs root privileges to even read the files to the archive. This makes rootless image manipulation not practical (and makes it effectively impossible to include umoci as part of a build system).

To fix this we should implement some sort of user mapping into the new version of unpack (which is saved in the unpacked bundle) and then on repack we use the same mapping to modify the *.mtree diffs we got.

[cyphar]

This is necessary for sane integration tests in #12.

[cyphar]

This is being worked on in https://github.com/cyphar/umoci/tree/packing-add-id-mappings.