Calculate ipfs identifier

[kpcyrd]

As part of the chksums::Hasher struct, we could also attempt to mimic what ipfs add -n does, ideally using the same chunker settings so it matches the output of an ipfs add with default settings.

This would then allow whatsrc to have tar download links with the help of an ipfs gateway (although the file is going to be unavailable unless somebody steps in to host it on their node).