allow pointer_based_stl_iterator to conform to the contiguous_iterator concept if we are in c++20

The changes presented here are rather trivial, and only allow the objects whose iterator type is pointer_based_stl_iterator to better work with ranges, in particular, the range resulting from code like

Array<Double, Dynamic, 1> myarray(10);
auto myarrayslice = myarray | std::views::drop(2) | std::views::take(5);

to automatically conform to the contiguous_range concept, which allows convenience/QOL operations to be available e.g. converting to std::span.