Breaking out `RepositoryBase` and deps as unbuilt Gem.

New unbuilt Gem, `meldd_repository`, will initially contain `MelddRepository::Base` and `MelddRepository::StoreResult` classes as described in the unbuilt Gem's "project" [`README`](https://github.com/jdickey/new_poc/blob/7394fa8/lib/meldd_repository/README.md).