shared pointer constructed with the array's context and the value points to array[index].
The function has zero computation cost.
struct S { double e; } auto a = RCArray!S(10); a[3].e = 4; auto s = a.toRCPtrAt(3); assert(s._counter == 2); assert(s.e == 4);
struct S { double e; } auto a = RCArray!S(10).asSlice[5 .. $]; a[3].e = 4; auto s = a.toRCPtrAt(3); assert(s._counter == 2); assert(s.e == 4);