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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.

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