Added copy constructors to "Programming in D"

[Walter Bright]

On 2/10/2022 12:06 AM, Mathias LANG wrote:
> I think an *immediate* improvement we could make to ease people's life is to 
> make `auto` peel the outermost qualifier level inside functions.
> 
> So that:
> ```D
> const int* ptr;
> auto p2 = ptr;
> static assert(is(typeof(p2) == const(int)*));
> ```
> 
> I really can't think of any downside to it, only upsides:
> - It is still predictable / consistent;
> - It *might* reduce the number of template instantiations in some cases;
> - It just flows more naturally... If you want full constness, there's still 
> `const`;

It sounds sensible to me.