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As detailed in comments a few commits ago, this is part of a big, big
roll towards keeping linking details far enough off to one side that
one can actually use most of the IPLD system without forming an
explicit compile-time dependency on any linking features (until, of
course, one uses the linking features).

This is a surprisingly small diff, because... well, because most of
the *interesting* features around linking simply weren't implemented
yet, and at this point everything that is has already been isolated
in the new cidlink and related encoding packages.
"CID" was *already* just a semantic placeholder that meant "eh, link".
Signed-off-by: Eric Myhre <hash@exultant.us>