refactor order circuit

[lispc]

decouple order id and merkle tree idx

Order id is like nonce, always incremental. User signed this id.

Meanwhile trade circuit uses idx of order merkle tree to fetch the order info then check trade price/amount against order info.

They are not same thing. After decoupling these two items, we can implement this #104

rewrite the order state transition login

These are the only valid transition methods:

1. from empty slot to a new order
2. from an old slot to a new order. Order with bigger id is newer.

The above two can be combined if we assume order_id:0 means empty order, real order starts with order id 1.