Move build-offsets and build-{labels-}elias-fano to their own binary
Users of swh-graph only need to run these commands to load old graphs; moving them to a dedicated binary spares them compiling the rest of the compression pipeline.
Staging instance, all changes can be removed at any time
Users of swh-graph only need to run these commands to load old graphs; moving them to a dedicated binary spares them compiling the rest of the compression pipeline.