add an EncodeWriter method, using the pooled marshallers

[MichaelMure]

This PR add an EncodeWriter method, to skip an allocation and copy, while still using the pooled marshallers.

[welcome]

Thank you for submitting this PR!
A maintainer will be here shortly to review it.
We are super grateful, but we are also overloaded! Help us by making sure that:

• The context for this PR is clear, with relevant discussion, decisions
  and stakeholders linked/mentioned.

• Your contribution itself is clear (code comments, self-review for the
  rest) and in its best form. Follow the code contribution
  guidelines
  if they apply.

Getting other community members to do a review would be great help too on complex PRs (you can ask in the chats/forums). If you are unsure about something, just leave us a comment.
Next steps:

• A maintainer will triage and assign priority to this PR, commenting on
  any missing things and potentially assigning a reviewer for high
  priority items.

• The PR gets reviews, discussed and approvals as needed.

• The PR is merged by maintainers when it has been approved and comments addressed.

We currently aim to provide initial feedback/triaging within two business days. Please keep an eye on any labelling actions, as these will indicate priorities and status of your contribution.
We are very grateful for your contribution!

[MichaelMure]

Beyond that, I'd like to suggest the following to address the // TODO: rename` and the inconsistent naming:

// DumpObject marshals any object into its CBOR serialized byte representation
// Deprecated: user Encode instead.
func DumpObject(obj interface{}) (out []byte, err error) {
	return marshaller.Marshal(obj)
}

// Encode marshals any object into its CBOR serialized byte representation
func Encode(obj interface{}) (out []byte, err error) {
	return marshaller.Marshal(obj)
}

I can add it in this PR or make another one if you find that suitable.

[rvagg]

👍 you're welcome to do the deprecation too and we can cut a new release; it'll be painful for anyone running strict linting tho