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fix staticcheck warnings

This should be the last piece to make our new CI workflows happy.

A number of codegen changes have happened recently. Most notably, basic
types behind a Maybe no longer use pointers, which means far fewer
allocations in common scenarios.

This results in a nice speed-up for go-merkledag's Roundtrip benchmark:

	name         old time/op    new time/op    delta
	Roundtrip-8    4.24µs ± 3%    3.78µs ± 0%  -10.87%  (p=0.004 n=6+5)

	name         old alloc/op   new alloc/op   delta
	Roundtrip-8    6.38kB ± 0%    6.24kB ± 0%   -2.26%  (p=0.002 n=6+6)

	name         old allocs/op  new allocs/op  delta
	Roundtrip-8       103 ± 0%        61 ± 0%  -40.78%  (p=0.002 n=6+6)

Run gofmt too, as prime's gen does not just yet.

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.

An io.Writer, by definition, will always copy bytes. That's fine in
general, and can't be worked around without breaking the writer's
contract.

However, the main use of go-codec-dagpb today is go-merkledag, which
simply uses the codec to encode a node into a buffer.

So, we had to create a new bytes.Buffer, write to it, and grab its
bytes. This is one extra allocation (the bytes.Buffer object itself),
plus copying the encoded bytes an extra time, since we must copy from
Encode's internal buffer to the bytes.Buffer.

Add a lower-level append-like AppendEncode that cuts the middle man,
removing both of those extra pieces of work.

For the sake of consistency, we add DecodeBytes to mirror the above on
the decode side. Decode already had a shortcut for Bytes, but this way
it's more evident what we're doing, and we also avoid allocating a
bytes.Buffer just to call Bytes on it.

Using these new APIs in go-merkledag shows nice results:

	name         old time/op    new time/op    delta
	Roundtrip-8    4.27µs ± 0%    4.07µs ± 0%  -4.50%  (p=0.004 n=5+6)

	name         old alloc/op   new alloc/op   delta
	Roundtrip-8    6.86kB ± 0%    6.38kB ± 0%  -6.99%  (p=0.002 n=6+6)

	name         old allocs/op  new allocs/op  delta
	Roundtrip-8       106 ± 0%       103 ± 0%  -2.83%  (p=0.002 n=6+6)

While at it, we formally deprecate Marshal and Unmarshal, since we're
starting to have lots of redundant API surface.

	name         old time/op    new time/op    delta
	Roundtrip-8    4.35µs ± 1%    4.27µs ± 0%  -1.92%  (p=0.004 n=6+5)

	name         old alloc/op   new alloc/op   delta
	Roundtrip-8    6.86kB ± 0%    6.86kB ± 0%  +0.01%  (p=0.004 n=5+6)

	name         old allocs/op  new allocs/op  delta
	Roundtrip-8       112 ± 0%       106 ± 0%  -5.36%  (p=0.002 n=6+6)

Like the previous commit, this helps reduce allocations as well as
improve performance thanks to the well-optimized protowire package.

And, as before, we get to remove unnecessary code.

	name         old time/op    new time/op    delta
	Roundtrip-8    4.81µs ± 0%    4.35µs ± 1%  -9.59%  (p=0.004 n=5+6)

	name         old alloc/op   new alloc/op   delta
	Roundtrip-8    7.14kB ± 0%    6.86kB ± 0%  -3.83%  (p=0.000 n=6+5)

	name         old allocs/op  new allocs/op  delta
	Roundtrip-8       119 ± 0%       112 ± 0%  -5.88%  (p=0.002 n=6+6)

IPLD's codec helper reader has a relatively high cost, unfortunately. It
was the main contributor to a slowdown in go-merkledag when moving from
the old protobuf gogo-generated decoder to go-codec-dagpb.

Using a []byte also means we can reuse protobuf's well-optimized "wire
encoding" helpers, which gets us extra speed and allows removing some
code.

This should not matter in practice for the time being, as the only
go-codec-dagpb user is go-merkledag and it uses bytes.Buffer everywhere.

In the future it would be nice for go-codec-dagpb to be just as
efficient with a stream decoder, but right now I don't have the extra
week to get into that. Plus, if the core protobuf implementation works
on []byte, one can assume it's reasonable for us to do the same.

Using the new BenchmarkRoundtrip in go-merkledag with go-codec-dagpb, we
get a significant uplift in performance:

	name         old time/op    new time/op    delta
	Roundtrip-8    6.49µs ± 1%    5.34µs ± 1%  -17.74%  (p=0.002 n=6+6)

	name         old alloc/op   new alloc/op   delta
	Roundtrip-8    8.07kB ± 0%    7.50kB ± 0%   -7.04%  (p=0.002 n=6+6)

	name         old allocs/op  new allocs/op  delta
	Roundtrip-8       171 ± 0%       148 ± 0%  -13.45%  (p=0.002 n=6+6)

std's errors has had wrapping for years now.

update .github/workflows/automerge.yml: don't restrict the copy workflow to .github/workflows/ (#24)

update .github/workflows/autorebase.yml: don't restrict the copy workflow to .github/workflows/ (#22)

We're a semver v1, so we can't remove exposed APIs unless we bump to
v2+. Bumping the major version to just rename two APIs seems a bit
overkill; keeping the old ones around and deprecating them is much
easier.

This partially reverts commit b8473079.

At least go-merkledag and go-graphsync still need a prototype chooser,
and go-ipld-prime-proto provides one, so this simplifies integration
with downstreams.

It's worth noting that we just add PBNode here, while
go-ipld-prime-proto also added RawNode. This is because this dag-pb
module leaves raw nodes out, since those are now in
go-ipld-prime/codec/raw.

The logic is just three lines, but it's still easier to not have to
copy-paste these between different projects.

make compatible with ipld-prime linksystem changes

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.

The module was defined as github.com/ipld/go-codec-dagpb/dagpb, but the
repository is at github.com/ipld/go-codec-dagpb. This means that neither
can be resolved as a valid Go package import path; the former results in
a GitHub 404, and the latter results in a valid GitHub repository which
holds a different module.

Rod confirmed that the /dagpb suffix wasn't intentional, so just get rid
of it.

While at it, mod tidy.