Forbid non-cryptographic hashes within /ipfs namespace

**Update:** This is DONE, we currently block non-cryptographic hash functions at blockstore level – see https://github.com/ipfs/go-ipfs/issues/4371#issuecomment-400031808

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It just occured to me that by supporting non-cryptographic hashes (such as murmur3) in multihash, these are also available for CIDs, and can thus be used within the /ipfs namespace (example: ipfs/ipfs-companion#305).

This is not good! /ipfs paths MUST be cryptographically verifiable, it's their primary property. A lack of cryptographic verifiability seriously breaks so many assumptions on so many levels.

This would probably be appropriate to enforce within the respective IPLD modules.