make Circle a class, not a trait
move all dimension2 stuff back into one package

Everything was importing everything else, and I think it makes sense for
all dimension2 stuff to stay cohesive. And trying to keep up that
convention for packages line line/circle/etc was feeling contrived.
remove Line classes

This just needs to be completely rethought.
move Angle-related classes into Angle.scala
move Vector types into Vector.scala
add Sign, SemicircleRadians.toCircleRadians
move Angle implicits from package to companions
start of a better Line/Ray design