Revert "cosmetics: getting rid of unused code."

This reverts commit 050c6b3c9920526d44d3e8589b185087cef1e5d4.

Conflicts:
	src/Exec.hs
feature: added Control.Monad.Utils.unitlM
cosmetic: OrderBook: don't expose OrderBook constructor
feature: reimplemented initOrderBook

It's not working properly though