This patch implements the changes described in
filecoin-project/FIPs#1004.

Unfortunately, go-f3 now needs to be aware of the internal structure of
`TipSet` objects as it sends them over the wire in one format, but signs
another. This effectively reverts #149.

Specific changes:

1. On the wire (and in finality certificates), `TipSet` objects are now
   directly cbor-marshaled within the payload instead of first being
   marshaled to byte strings.
2. Instead of directly covering the entire tipset list with the payload
   signature:
  1. `TipSet` objects are "marshaled for signing" per the FIP change
      proposed above.
  2. These marshaled tipset objects are stuffed into a merkle tree.
  3. The merkle tree root is signed (along with the payload's other
     fields), again according the proposed changes.

fixes #166

Into a single Signatures interface.