Include prefix in Multihash.toString()

[vorburger]

ToDo:

• Test coverage

[odisseus]

The premise of this PR is completely wrong.

First, Qm isn't the right prefix for Base58. In fact, no valid Multibase string can start with Q. See specification, implementation.

The hashes that start with Qm are typical (though not required) for IPFS, and they actually don't include the Multibase prefix. See discussion.

Second, the method Base58.encode from io.ipfs.multibase produces results which already include the base prefix. See test examples.