list: add features from frg::intrusive_list to frg::list

[rdmsr]

I added feature parity between frg::list and its backing frg::intrusive_list.

The only "issue" which may be unexpected is that ->object is required to access the underlying object when doing a range-based for loop:

frg::list<int, allocator> list{};
list.push_back(1);

for(auto elem : list) {
   std::cout << elem->object; << std::endl; // prints 1
}

[qookei]

I'd probably be better to implement an iterator type that directly returns the list item and not the intrusive list wrapper type, since that's an implementation detail.