BREAKING CHANGE: dag-cbor nodes now represent links as CID objects

The API for [dag-cbor](https://github.com/ipld/js-ipld-dag-cbor) changed.
Links are no longer represented as JSON objects (`{"/": "base-encoded-cid"}`,
but as [CID objects](https://github.com/ipld/js-cid). `ipfs.dag.get()` and now always return links as CID objects. `ipfs.dag.put()` also expects links to be represented as CID objects. The old-style JSON objects representation is still
supported, but deprecated.

Prior to this change:

```js
const cid = new CID('QmXed8RihWcWFXRRmfSRG9yFjEbXNxu1bDwgCFAN8Dxcq5')
// Link as JSON object representation
const putCid = await ipfs.dag.put({link: {'/': cid.toBaseEncodedString()}})
const result = await ipfs.dag.get(putCid)
console.log(result.value)

```

Output:

```js
{ link:
   { '/':
      <Buffer 12 20 8a…> } }
```

Now:

```js
const cid = new CID('QmXed8RihWcWFXRRmfSRG9yFjEbXNxu1bDwgCFAN8Dxcq5')
// Link as CID object
const putCid = await ipfs.dag.put({link: cid})
const result = await ipfs.dag.get(putCid)
console.log(result.value)
```

Output:

```js
{ link:
   CID {
     codec: 'dag-pb',
     version: 0,
     multihash:
      <Buffer 12 20 8a…> } }
```

See ipld/ipld#44 for more information on why this
change was made.