Fixes and additions via HexKit 1.1

Fixed edge case bug in IsWithinRectangle.

Rewrote GetHashCode. It was bad and I feel bad.

New instance methods: HalfSextant, IsOnCartesianLine,
IsOnCartesianLineSegment

New static methods: CartesianRectangleBounds, IsSameRotationIndex,
NormalizeRotationIndex