Remove libipld dependency

[vmx]

Description

libipld is deprecated, hence remove its usage. Instead use serde_ipld_dagcbor and cid.

Closes #620.

Other changes

This commit also removes the to_block() and from_block() functions. They are part of the public interface, so this is a breaking change. To me it's unclear what are used for. The only occurrence I found on GitHub was in the Kepler code base, but that project was archived already a year ago. Hence I decided to remove that functionality. It case it's still needed, I'm happy to add it back in case some more information is provided on how that API should be used.

Tested

I ran cargo test within the ssi-ucan crate.

[CLAassistant]

All committers have signed the CLA.

[bumblefudge]

hehe i vouch for this weirdo. @chunningham nota bene on the kepler side.