Proof of Staking

[Lawliet-Chan]

We need proof-of-staking for yu, should develop a tripod named pos for it.
This tripod has the following Writings:

• Staking
  Callers stake their tokens on the chain. Must check if their account balance is sufficient.
  Then

   assets.SubBalance(caller_addr, token_amount)
   staking.State.Set(Staking, caller_addr, token_amount)

• Unstaking
  Callers unstake and withdraw their tokens.

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• Delegate
• Undelegate