Implement merging nearby turns together.

When there are multiple turns near each other (< 60m), set the lanes
that are allowed on earlier turns such that staying on that lane will
carry you through the other turns.

For a standard left/right turn at a traffic light at two one-way roads
(dual carriageway), this means that the left and right turns are
correctly highlighted.
Previously, the code was such that if `kl` was true, that meant that there was a road going to the right, and that we had to keep left, and if `kr` was true, that meant that there was a road going to the left, and that we had to keep right. However, if there are roads going to the left and to the right (and straight), both `kl` and `kr` would be true. This case wasn't handled correctly (I believe it would say to keep right). Such a turn is [here](http://www.openstreetmap.org/#map=19/32.89642/-97.03849).

This code checks to see if both are true, and if so, specify that we're continuing straight.