Re-exports from ipfs

[koivunej]

Currently it's a bit of a mess. There are some parts of libipld re-exported but that is not enough and we need to depend on libipld directly on http and examples. I noticed this when I needed to get a working revision of rust-ipld and I needed to upgrade the rev to three places in #147. Intentionally trying to do as little of this in #147 so just setting the rev's on the dependencies in Cargo.tomls.

Not exactly sure what would be the correct way forward. Since the libipld is used on public api I think we should just export it, maybe even completly under for example ipfs::ipld.

Cid and cid::Codec are also re-exported but not cid::Error which is a bit strange. I think the leaning towards re-exporting it under ipfs::cid.

For libp2p things are pretty ok at the moment. For some reason libp2p::core::ConnectedPoint is re-exported even though it's only used under src/p2p/swarm.rs. Multiaddr, PeerId, PublicKey, Keypair seem like useful to have. Should these also be under ipfs::libp2p...? I don't think so, but I guess we'll learn more when someone wants to use a custom combination of some rust-libp2p features with the code from ipfs.

[cdata]

FYI in Git HEAD only Cid is re-exported. cid::Codec is not, and this makes it very hairy to construct a Cid from outside the crate.