Making RCSlice and DIP74 work with const and immutable

[Jakob Ovrum via Digitalmars-d]

On Sunday, 1 March 2015 at 15:40:06 UTC, Atila Neves wrote:
> I've lost count now of how many times I've had to downgrade to 
> auto despite always wanting immutable or const. This doesn't 
> work:
>
>  auto reg = regex(`(foo)`);
>  const match = "foo".matchAll(reg);
>  writeln(match.captures); //oops, captures isn't const
>
> It should, but it doesn't. Maxime talked about it here as well:
>
> http://pointersgonewild.com/2014/07/11/the-constness-problem/
>
> Atila

`match.captures` is a range; it's only natural for a range to 
have mutable state to be iterable.

D's const is a bridge between immutable and mutable. const has to 
be transitive because immutable is transitive. Don't use it as if 
it was C++ const: there's no logical const in D, and if there 
ever will be, it can't use the same `const` type qualifier.