Simplify creating constructors

[metacosm]

The syntax is quite arcane and is difficult to get right, in particular when needing to call super constructors.

[gsmet]

I think we need some general infrastructure to easily generate proxy methods, be it constructors or regular methods.

[dufoli]

hmm, I have seen the message on zulip and Thinking to implement few stuff. Indeed I face same issue with cxf ;-)
wdyt about adding in ClassCreator:

    public MethodCreator getConstructor(String... parameters) {
        return getMethodCreator(MethodDescriptor.ofConstructor(className, parameters));
    }

    public MethodCreator getSuperConstructor(String... parameters) {
        return getMethodCreator(MethodDescriptor.ofConstructor(superClass, parameters));
    }

[dufoli]

I implement a part in #69 and another in #68