We shouldn't implement CBOR functions on the "abstract" miner info. Otherwise,
we could end up trying to actually _use_ this "abstract" info when decoding
state (which won't work across version).
Also remove the use of these CBOR functions, and instead explicitly use miner0
types. We'll have to abstract over versions eventually, but we'll probably need
some form of abstract miner builder (or maybe even adding some "add sector",
etc. functions to the current miner abstraction?
46f5b62a76