Store maximum uint for fully filled or cancelled orders

[abandeali1]

Summary

This is a simple modification to the filled and cancelled mappings that would allow fully filled or cancelled orders to be easily detectable on-chain.

Motivation

There is currently no way to tell if an order has been fully filled or cancelled on-chain with only a reference to that order's orderHash. As is, the order takerTokenAmount must be known and verifiable (all of the order parameters must be used to verify this on-chain).

Specification

When an order is fully filled or cancelled, the maximum unsigned integer (2**256 - 1) can be stored in the corresponding filled or cancelled mappings.

Rationale

This change could enable interesting features down the line, such as conditional orders (see #9 ). Note that orders that have been both partially filled and cancelled will not be detectable using this proposal. However, it can always be used to tell if an order has not been fully filled/cancelled.