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We recently updated the spec to say that decoders "should" allow
decoding a PBNode in either order of its fields. That is because some
IPFS data out in the wild is already encoded in the non-canonical
format, which is what Protobuf uses as a default, too.

This change makes the decoder here comply with the spec change, adding a
test with the encoded block that caused this entire spec change in the
first place: go-ipfs/test/sharness/t0110-gateway-data/foofoo.block.

The change to the decoder is slightly subtle, because the decoder used
to leverage the knowledge that Links must come before Data to start the
Links list early, and finish it as soon as Data arrives.

Since that order is now unknown, we must have some extra logic to
support either order. We also need special code to make sure Links is
always set, just like before.

We also add a test to double check that Data between Links is rejected.

A similar change to Encode and Decode function names came in
go-ipld-prime v0.9.0, so this keeps this library in sync.

Improved package docs a bit to talk about the new go-ipld-prime
LinkSystem, which is a bit clearer to refer to than before.

Target-of-opportunity fix some typos in docs.

Fixes: https://github.com/ipld/go-codec-dagpb/pull/3