Collective signature (a.k.a CoSi)

**Describe the solution you'd like**
Implement collective signature protocol (a.k.a [CoSi](http://arxiv.org/abs/1602.06997)) through following messages

- [ ] `CosiCommit` asks device to commit to CoSi signing
- [ ] `CosiCommitment` in reply to the former
- [ ] `CosiSign` request to sign using CoSi
- [ ] `CosiSignature` retrieves signature

**Describe alternatives you've considered**
`CosiSignature` may be replaced with a generic `Buffer` message with a numeric reason value, but this would duplicate protobuffer selection by message type.

**Additional context**
Requires fibercrypto/skywallet-protob#80