Add power table commitment (CID) and accessory commitment to Tipset structure

[anorth]

Add a power table commitment and a placeholder for an accessory commitment into the Tipset structure. Both are opaque to GPBFT but must be included in considerations of equality and signing payloads (wait for #149 first).

See FIP PR filecoin-project/FIPs#983

[anorth]

The tipset structure used within gpbft was replaced with an opaque byte array. We'll probably need a new structure defined further out in the integration layer, to which these fields are added.