ENH: Treat stop and limit prices differently when rounding.

Stop and limit prices both trigger when a price crosses some threshold, but
they trigger in "opposite directions".  For example, on a buy, a limit price is
triggered when a price falls below a specified value, whereas a stop price
triggers when the price exceeds a specified value.

Our current stop/limit price rounding logic is asymmetric, preferring to "round
to improve" the specified price.  This change makes it so that we interpret
"improvement" in opposite directions for stop vs limit prices.