Limit Cancel

[crnbarr93]

Closes: #10

What is the purpose of the change

This PR adds the ability for a user to cancel their limit order by providing the book ID, tick ID and order ID. The following process takes place. To cancel an order the following criteria must be met:

• Order must exist
• Sender must be order owner
• No funds can be sent with the cancel order

An automatic refund is sent to the owner upon cancelling an order. Upon cancelling an order the liquidity for the given tick is also deducted by the amount in the order, if the tick's liquidity becomes 0 the tick is removed.

Testing and Verifying

This change added tests and can be verified as follows:

cargo unit-test test_cancel_limit

• Tests can be found in tests/test_order.rs
• Small adjustments were also made to test_place_limit to account for liquidity tracking

[AlpinYukseloglu]

LGTM, nice work! Made a couple light related driveby changes, lmk if you'd like to revert any of them.