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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)

std's errors has had wrapping for years now.

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