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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)