* This is 99% mechanical:
Extracted repo.Repository interface that only exposes high-level object and manifest management methods, but not blob nor content management.
Renamed old *repo.Repository to *repo.DirectRepository
Reviewed codebase to only depend on repo.Repository as much as possible, but added way for low-level CLI commands to use DirectRepository.
* PR fixes
Also introduced strongly typed content.ID and manifest.ID (instead of string)
This aligns identifiers across all layers of repository:
blob.ID
content.ID
object.ID
manifest.ID
