path: add ToCid API

[schomatis]

Closes ipfs/go-cid#130. (This will need to be manually closed since it's in a different repo. seems this works)

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[schomatis]

Note this. I'm not sure what's the expected IPNS format; we might need to validate its CID either way.

[hacdias]

This repository is no longer maintained and has been copied over to Boxo. We are now in the process of reviewing issues and PRs and moving the ones that are still relevant. This issue has been merged with a few other issues, and moved to ipfs/boxo#198.