json,cbor: switch to "ipld" tag (#244)
json,cbor: switch to "ipld" tag (#244)

This change moves us toward the "ipld" tag meaning something like
"can logically be used as a codec code in a CID";
with the recognition that this is a bit squishy and there is some amount
of gentle abuse around the edges of CIDs which we willingly turn a blind eye to.

Ref: https://github.com/multiformats/multicodec/issues/242
Ref: https://github.com/ipfs/go-ipfs/issues/8471
multiformats/multicodec