PiggyBankSmartContract

function: Deposit
parameters: Amount, Deadline
code:
    - Check if function Caller transfer asset to this contract by at least the Amount.
        - if not, reject transaction
    - Increase function Caller's balance by Amount.
    - Lock this chunk of asset with the deadline.

function: Withdraw
parameters: Amount
code:
    - Check if function Caller's balance is at least Amount
        - if not, reject transaction
    - Check if there's at least an Amount worth of assets unlocked by this time.
        - if not, reject transaction
    - Re-calculate locked assets.
    - Decrease function Caller's balance by Amount.
    - Transfer an Amount of asset to function Caller.