Make U-Turn a type (U-Turn left vs U-Turn right)

[MoKob]

Right now, u-turn is a direction modifier, rather than a turn type itself.

We should consider making uturns a turn-type:

• we could then indicate whether a u-turn is to the left or to the right (consider left-sided driving, or maybe special cases)
• we could free up the uturn modifier to be invalid and to better describe invalid turns
• having a turn-type for u-turns removes ambiguity for having turn uturn and continue uturn.

Due to the removal of uturn as a modifier, and since it's not really well announed as a turn, this change has to be considered an API change and could only be implemented with the next major version.

[TheMarex]

The original need for this was due to left/right hand driving. This was fixed by adding driving side, so I don't think we need this anymore.