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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.