Support fungible assets in `buy`

[grabbou]

trafficstars

Could it be possible for buy() to support tokens and/or price to support purchasing fungible assets? For example, there's 5 assets listed for a total price of 5. It should be possible to buy a given number of assets (price would be automatically calculated in such case).

Right now, alternative approach is to create bid and execute sale separately, which is pretty much what buy does in a single transaction.