Add arcCCW and friends.  Fix offset joins.

These variants for arc creation hide the complexity of angles and
orderings between them and instead always gives an arc in a fixed
direction (clockwise "CW" or counterclockwise "CCW") from some start
angle or direction to an end angle or direction.

There is a possibility that `arcDCCW` and `arcDCW` could be constrained by `HasTheta` instead of a direction, but that broke inference for me and I didn't play around with it after that.