MultiPack index support

[Byron]

MultiPack indices are saving resources and speed up object access, they are less common locally but great on the server.

Tasks

• multi_index::File as a way to read the file format
• multi-index verification #282
• multi-index writing
• integration into git_odb::Store
• assure packs can be built when a multi-pack index is present
• optional light-weight verification similar to what git does