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We now have CIDs support!  You can create links backed by cids,
and marshal them with dag-cbor; and you can unmarshal cbor data
with dag-cbor and expect things with the CID link tag to be parsed
into CIDs and exposed as IPLD Links.  Yay!

(Dag-json is lagging.  The parse for those links is... more involved.
When supported, it'll similarly have its own unmarshal and marshal
just like the ones this diff introduces for dag-cbor.)
Signed-off-by: Eric Myhre <hash@exultant.us>