Invisible IPLD nodes

How should we deal with degenerate IPLD nodes that are *just* CIDs?

```
c := dag.add("value")
b := dag.add({"/": c })
a := dag.add({"/": b})
```

The question is: what do `/ipld/$a`, `/ipld/$b` resolve to?

Option 1: `/ipld/$a` resolves to `{"/": b}` (same for `b`). This is inconsistent with the rest of IPLD because it's usually impossible to resolve a path to a CID.

Option 2: `/ipld/$a`, `/ipld/$b`, and `/ipld/$c` all resolve to `"value"`. This would make `a` and `b` disappear. Furthermore, a blockstore may *have* `$a` but be unable to resolve `/ipld/$a`.

Option 3: Disallow them. That is, valid IPLD nodes may not be bare CIDs.

Thoughts?