This removes the Export method in nar-bridge, and updates all users to
the version now in storev1pb.
It moves the roundtrip test to the importer crate, and some of the
utility functions into a separate util_test.go file.
Change-Id: I81d9e0b35dfd78ef1042bed307281eecd2aaa2a8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9603
Reviewed-by: Brian McGee <brian@bmcgee.ie>
Tested-by: BuildkiteCI
