remove reference
The Reference struct has been moved into the oci-spec crate.
This PR removes the duplicated files and exports Reference from the oci-spec crate.
Consumers of oci-client won't even need the change, unless they were importing Reference from oci_client::reference::Reference instad of oci_client::Reference.
On top of that, I've also addressed some clippy warnings.
CC @cgwalters
Hey folks, will a new release be published after this PR gets merged?
@thomastaylor312 would you mind giving a quick look at this PR?
@Xynnn007 yes, I think we can do that
@thomastaylor312 no worries!