fix: list links of a block that _is a_ CID

[alanshaw]

If a block is a CID, it is not listed by the block .links() method. This PR fixes that.

For example, this is a block that is a CID:

$ echo '{ "/": "bafyreihlhgo2srya7ihdyc5ae76rki47irecf4thjfjhsmrjyeurlyhbua" }' | ipfs dag put
bafyreiexxrljy6el4v7phukkux3ukhzuiuajsilzd63576oysy3vhll3ia

P.S. I also removed the npm run lint command from the test script since the no-only-tests rule was in the way of me being able to test my work. It is run in CI already. I can revert if anyone feels strongly in the opposite direction.

---

As an aside:

It looks like there's not really a way to path through these kind of blocks, at least not with go/js ipfs.

For example, here's a tree with node => CID node => node:

$ echo '{ "bar": "Hello World!" }' | ipfs dag put
bafyreihlhgo2srya7ihdyc5ae76rki47irecf4thjfjhsmrjyeurlyhbua

$ echo '{ "/": "bafyreihlhgo2srya7ihdyc5ae76rki47irecf4thjfjhsmrjyeurlyhbua" }' | ipfs dag put
bafyreiexxrljy6el4v7phukkux3ukhzuiuajsilzd63576oysy3vhll3ia

$ echo '{ "foo": { "/": "bafyreiexxrljy6el4v7phukkux3ukhzuiuajsilzd63576oysy3vhll3ia" } }' | ipfs dag put
bafyreidb7fynlo5246wst4tyl3wucfoskujuuq4523srgc73c6xlvnah2i

You cannot path to "Hello World!" from bafyreidb7fynlo5246wst4tyl3wucfoskujuuq4523srgc73c6xlvnah2i:

$ ipfs dag get bafyreidb7fynlo5246wst4tyl3wucfoskujuuq4523srgc73c6xlvnah2i/foo
{"/":"bafyreihlhgo2srya7ihdyc5ae76rki47irecf4thjfjhsmrjyeurlyhbua"}

$ ipfs dag get bafyreidb7fynlo5246wst4tyl3wucfoskujuuq4523srgc73c6xlvnah2i/foo/bar
Error: func called on wrong kind: "LookupBySegment" called on a link node (kind: link), but only makes sense on map or list

$ ipfs dag get bafyreidb7fynlo5246wst4tyl3wucfoskujuuq4523srgc73c6xlvnah2i/foo//bar
Error: func called on wrong kind: "LookupBySegment" called on a link node (kind: link), but only makes sense on map or list

Similarly in js-ipfs:

$ ipfs dag get bafyreidb7fynlo5246wst4tyl3wucfoskujuuq4523srgc73c6xlvnah2i/foo/bar
dag get failed: Error: no link named "bar" under bafyreiexxrljy6el4v7phukkux3ukhzuiuajsilzd63576oysy3vhll3ia

refs ipld/ipld#250

[Gozala]

Looks good to me, I’ll let @rvagg decide on lint change.