refactor: Make Repo independent

[dariusc93]

In the current implementation, Repo is initialized and used while initializing Ipfs, making it require for repo and dag operations, however there may be instances where such operations should be separate (eg only needing to load blocks locally without initializing Ipfs or needing it to be online, etc).

This PR separates Repo out to be independent of Ipfs but can still be used within an instance when calling UninitializedIpfs::set_repo. Repo can also be used with IpldDag without Ipfs, and unixfs functions, however online operations will not work unless Ipfs is initialized and used.