chore(ipld): switch to using top-level ipld-prime codec helpers

[rvagg]

Ref: ipld/go-ipld-prime#428

since the bindnode usage here gets a few recommendations as an example usage, this switches to the top-level encode/decode helpers to encourage that as best-practice (one big benefit is that we can avoid the whole Representation() thing on encode). Also less code, so yay.

[hannahhoward]

Approved. Can we go a step further and simply remove these functions?

[rvagg]

Yeah, I'll have a go. They're mainly helpful now for abstracting away the use of dagcbor, which is pretty trivial I suppose, they're all single-line functions.

[rvagg]

This is the only awkward bit of removing ipldutil.go entirely - it preloaded dagcbor and raw codecs; but there are some places that use testchain that don't touch the codecs directly but they're encountered by the loader because testchain uses a dagcbor cidlink.LinkPrototype.
But I think it's not unreasonable to add this here since it's also referencing cid.DagCBOR in this file.

[hannahhoward]

sounds fine. oy. I wonder if we should just preload them in the root package at some point

[rvagg]

my initial take was to put them both there - but when you use graphsync as a whole (not just as a unit test) you're going to get dagcbor no matter what because it's in the messaging layer. raw is going to come if you need it, but that's up to the user IMO, maybe they want to add dagjson, dagjose, git, and more, but that's up to them so I'm fine with localising this I think.

[hannahhoward]

cool.

[rvagg]

Removed ipldutil.go entirely, pretty clean