Add abstracts for Degrees/Radians

[bendmorris]

Adds type safety around working with degrees/radians, and eliminates the need to convert explicitly via * MathUtil.RAD etc.

var degs:Degrees = 180;
var rads:Radians = degs;
trace(rads); // "-1.0*PI"

Breaking as Degrees can't be used in place of a Float, so Math.sin(angle * MathUtil.RAD) usage will need to be updated to use a cast or Degrees.sin.