Avoid Boxing for Constructor Invocations

[matthias-springer]

When calling a generic constructor factory (the generated methods that start with _new), all parameters of type of a type variable are boxed implicitly. This should be avoided. Instead, get rid of the generic factory and call a specialized constructor directly (if it is fixed as described, it would probably also fix #4).

This only applies to constructors but not factories (Constructor factory != factory constructor)! Generic method specializations (#6) could be used to get rid of boxing when calling a factory constructor.

E.g.:

class A<T> {
    T variable;
    A(this.variable);
}

new A<int>(42);   // 42 is boxed