Add a fetchIpfs function

[Chris Warburton]

Takes a hash. It should look for a /ipfs directory, if not found it
should look for a localhost HTTP API and fall back to ipfs.io.

Find some well-known content that we can use as our check.

Note: we don't need to take an sha256 or anything, since IPFS is
content-addressed.

See if we can make them fixed-output derivations.