Extract non-core functionaly into new package

[kevina]

go-cid is used by any package that does anything IPFS or IPLD related. we should probably move non-core functionally into a new package to avoid having to update so many packages when non-core functionality changes.

Code in the following packages should likely be moved:

• set.go and tests
• format.go and tests
• Everything in cid-fmt/.

[kevina]

@Stebalien you didn't want to extract set.go, what parts did you want to extract to go-cidutil? Do you want to move StreamingSet?

[Stebalien]

For now, let's leave set.go (well, the Set structure in set.go). However, feel free to move StreamingSet. Given that it's a "new" feature, feel free to "move" it by recommitting it to the new repo (just make sure to set the commit author to magik6k).

[Stebalien]

I would move format.go and cid-fmt (and the Identity cid builder).

[kevina]

Okay. Just how many packages use cid.Set?

The Identity cid builder will come imminently after the extraction and I will make a p.r. for it.

[Stebalien]

ipfs, bitswap, filecoin. I'm not really worried about breaking our code, I just don't want to mass-break other people's code (yet).