Update for compatibility with multiformats 4 configuration-free API

[andymatuschak]

multiformats/js-multiformats#36 rewrites the API of multiformats to avoid creating a central registrar object, but this module expects the now-defunct object as an argument to its constructor. This module will need to be rewritten either to import its own dependencies from multiformats or to consume them as arguments.

[rvagg]

Yep, should be a fairly quick job for someone. I've nearly done it myself over the last few days. Mikeal just did dag-cbor, the new dag-pb is done, this is the next piece of the puzzle.