Added copy constructors to "Programming in D"

[Meta]

Why do we even bother with `in` when we can do:

alias In(T) = const scope T;

void test(In!int n) {
     pragma(msg, typeof(n));
}

?

onlineapp.d(3): Deprecation: storage class `scope` has no effect 
in type aliases
const(int)

...oh