Further optimizations for random access/contiguous ranges

[foonathan]

Similar to the fused operation from #9, there are more optimizations possible for random access and contiguous ranges. It would be neat to have them considered.

For random access ranges, the iterator only needs to store a difference_type together with a virtual function reference at(difference_type n) corresponding to the operator[] overload of the iterator. Any traversal or comparison can then be done by integer operations on the difference type; virtual dispatch is only necessary for operator* and operator==(iterator, sentinel).

For contiguous ranges, the iterator only needs to store a pointer. All operations except operator==(iterator, sentinel) can be done without virtual dispatch. For contiguous common ranges in particular, the any_view::iterator should not have any overhead compared to span::iterator.

[patrickroberts]

Additionally, for contiguous, sized ranges, the any_view specialization can just return pointers from begin and end. 🙃

These are awesome suggestions and I'd love to get to them at some point, but probably not until after C++Now (unless someone wants to tackle any of these at library-in-a-week, in which case I will be there to help).