Initial support for `calc( ... )` Length.Calc (LengthUnit.Expression), evaluate on demand Add failing test for nested calc (`calc( ... + calc( ... ) )`) Add support for nested `calc()` statements Do GetPixels inside calc functions (for percentages) Division by zero throws correct DivideByZeroException Tests & support for invalid syntax, constants (e, pi, nan), scaling percentages Remove token count check, will need to re-think this validation later Rename test class to CalcTests, clean up Check if length unit is expression & evaluate on demand Implicit eval Pass parent property values into ToYoga() for calc