Circuit value improvements

[mitschabaude]

This fixes some issues with CircuitValue

• It no longer breaks when you declare a constructor with different argument structure than the base constructor. That's because ofFields and fromJSON no longer use new this(...) to instantiate the class
• There's now a CircuitValue.fromObject method, which is nicely typed (in contrast to the generic constructor). It expects an objects that has the same props as the CircuitValue, but without the methods.

[Trivo25]

a much needed improvement, makes working with CircuitValues a lot easier!