Adhere DEX order to limit order behaviour with multiple counter orders support

[greenhat]

Branched from #13

Challenge 1.
Figure out how "select" proper counter orders without a 'sort' op. Example:
For older buy order (price 5 ergs, total 15 ergs) which is matched with 2 newer sell orders (price 5, 2 tokens), (price 4, 2 tokens) how to start "selecting" tokens from the second sell order and finish with the first order?
Without the solution, we'd have to limit to 1-1 (single counter order) order matching per swap tx.

Todo:

• check that counter order are sorted by token price;
• draft contracts for multiple counter orders;

Continued in https://github.com/ergoplatform/ergo-contracts/milestone/1