Mixed assets in "market" operations.

[dorothy-zbornak]

Summary

Remove the restriction of all orders sharing the same maker or taker asset data in marketBuy...() and marketSell...() functions.

Motivation

The V3 world is evolving into one where the same asset can be represented by different asset data encodings. A good example would be with the new Bridge (#47) proxies, which can exchange any ERC20 token but can also carry arbitrary payloads in its asset data.

Specification

In V2, we copied the makerAssetData (in the case of a marketBuy...()) or the takerAssetData (in the case of marketSell...()) to each subsequent order passed into the market functions. This essentially enforced homogenous asset types across all orders.

Going forward, we will simply no longer perform this copy, and fill orders as usual, continuing
to track and assert the maker/taker asset amounts bought/sold.

Rationale

Because we no longer enforce homogenous asset types, this places more burden on the taker/interface to ensure that orders are, indeed, compatible or desirable.