Making RCSlice and DIP74 work with const and immutable

[via Digitalmars-d]

The RC wrapper allocates _mutable_ memory for the reference count 
on the heap (it has to do that anyway, because it needs to be 
shared by all instances). As far as I understand, mutating a 
mutable variable is safe if all users of that variable are aware 
that it's mutable, _even if it's only reachable through a const 
pointer_.

This condition is easy to enforce in an RC wrapper, it just has 
to keep the pointer to the refcount private. To make it easier to 
prove safety, the pointer needs to be declared as const; all 
refcount manipulation needs to happen through two small 
(inlinable) helper methods that do the appropriate cast() magic. 
The refcount can also be stored next to the payload (better for 
cache locality anyway), in which case we don't even need to store 
a const pointer at all, which also means that nobody can 
accidentally access it in the wrong way.