feat: add createNode() and createLink() factories

[rvagg]

To match DAGNode and DAGLink constructors of old

See the README notes for examples of how it would be used, it's really just a shorthand for making the objects but does so safely and reliably.

I also considered moving prepare and validate and these new functions on to a @ipld/dag-pb/util export rather than the main export, but 🤷.

[rvagg]

I think I'm pretty happy with this, it almost does away with the need for prepare as an export and simplifies code, reducing magic object properties. Will Update ipfs/js-ipfs-unixfs#116 with a commit using this to demonstrate.

[achingbrain]

🤷 I kind of liked that I was passing plain old js objects into dagPb.encode and not having to use classes or special functions that return something that may be non-obvious. It seemed like a simple and intuitive way of doing things to me.

The only improvement I would make is maybe not having it require a Links property.

I think prepare and validate could be removed, they haven't really been necessary for the multiformats migration work I've been doing.

[rvagg]

k I don't mind either way, as long as we get to strict forms in the end that's all I care about. If you want to back out the change to ipfs/js-ipfs-unixfs#116 I'm fine with that and may back out this change if it's not being used there.

[Gozala]

I literary meant this:

Suggested change

	return prepare({ Data: data, Links: links })
	return { Data: data, Links: links }

[rvagg]

The reason for the prepare() is simply to ensure the data types and shape are correct, do some basic coercion where possible and to throw an error if this won't pass encode(). It's essentially doing { Data: data, Links: links} but with checking too so I don't have to duplicate that logic. So the helpers are not just giving you correctly shaped objects but are checking the values you pass it too.

[Gozala]

And this

Suggested change

	return asLink({ Hash: cid, Name: name, Tsize: size })
	return { Hash: cid, Name: name, Tsize: size }

[Gozala]

Suggested change

	export function createLink (name, size, cid) {
	export function createLink ({ name, size, cid }) {

[rvagg]

The idea here was to provide something that had the same function signature as the old PBLink and PBNode constructors in ipld-dag-pb.

[Gozala]

Suggested change

	export function createNode (data, links = []) {
	export function createNode ({ data, links = [] }) {

[rvagg]

OK, so neither of you are happy with this, I'm not overjoyed myself. Should I just remove them entirely? @Gozala @achingbrain?

[achingbrain]

The only extra thing I want out of this module is to not require a Links property:

dagPb.encode({
  Data: Uint8Array.from([0, 1, 2])
})

I would even skip validation on encode entirely as it then guards on all the properties anyway - if the user cares they can call validate themselves. I think this is consistent with the yolo approach IPLD takes to validation in other places (ref: all the is-it-a-string-or-is-it-a-byte-array discussions).