worrisome behavior when upgrading refmt

Just wanted to note something we ran into in our codebase. We recently upgraded our refmt because it has a lot of performance improvements. However, we use uint64s in our objects. When you call https://github.com/ipfs/go-ipld-cbor/blob/master/node.go#L95 (`Decode`) here, those uint64s get unmarshaled into overflowed `int` and then re-encoded back... so the bytes and CID are different.

I realize the comment says that might happen, but this isn't "canonical cbor" we're talking about... this is a change from a uint64 into an overflowed Int that then gets reencoded.

The version *currently* in this library works fine... just wanted to give a "heads up" when you're thinking about upgrading.