Multihash support for root-manifests and ENS

[cobordism]

With the advent of encrypted swarm uploads (swarm up --encrypt) requiring double length hashes, it is time to update swarm to use multi-hash.

Adding the double-length encrypted swarm hashes to ENS would require a lot of changes. According to Arachnid:
"Presently the return type for content hashes is 'bytes32'. Changing this would require writing a new resolver (ideally, after writing an EIP describing the new type). Existing tools that use the resolver (including manager.ens.domains) would need updating to support the new type."

And if we are already in need of changing so much, this is the perfect opportunity to:

"Instead of bytes32, we can use 'bytes', and start using multihash, so we can support Swarm, IPFS, and any other content-addressed system."

[cobordism]

I guess we already have this open:

#186
#166