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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.

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.

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.