More versatile key design

[alxiong]

To improve versatility of our wallet, we might consider some of the following features:

1. hierarchical deterministic wallets
2. separation of proof generation from transaction authorization to enable delegation
3. separation of "staking key" from "spending key"
4. diversified addresses with consistent incoming viewing key
5. no platform-wise auditor/tracer, but only application specific ones
6. (optional) outsourced message detection (for receiver memos etc)

which requires modification of our current key design and the snark circuit.