[Feature request] Consider adding support of reference proxy

[tower120]

Continuing #47.
Currently, iterator_facade does not allow to have RandomAcess iterator with reference proxy (aka std::vector<bool>::iterator::reference) .

It looks like that the only thing that prevent this - is iterator-category deduction/override rules https://www.boost.org/doc/libs/1_68_0/libs/iterator/doc/iterator_facade.html :

iterator-category(C,R,V) :=
   if (C is convertible to std::input_iterator_tag
       || C is convertible to std::output_iterator_tag
   )
       return C

   else if (C is not convertible to incrementable_traversal_tag)
       the program is ill-formed

   else return a type X satisfying the following two constraints:

      1. X is convertible to X1, and not to any more-derived
         type, where X1 is defined by:

           if (R is a reference type
               && C is convertible to forward_traversal_tag)
           {
               if (C is convertible to random_access_traversal_tag)
                   X1 = random_access_iterator_tag
               else if (C is convertible to bidirectional_traversal_tag)
                   X1 = bidirectional_iterator_tag
               else
                   X1 = forward_iterator_tag
           }
           else
           {
               if (C is convertible to single_pass_traversal_tag
                   && R is convertible to V)
                   X1 = input_iterator_tag
               else
                   X1 = C
           }

      2. category-to-traversal(X) is convertible to the most
         derived traversal tag type to which X is also
         convertible, and not to any more-derived traversal tag
         type.

The simplest solution for now, seems to override manually iterator_category in Derived class.

---

Maybe add some proxy tag? Like:

class node_iterator
	: public boost::iterator_facade
	  <
		node_iterator
		, std::pair<int, int>
		, boost::bidirectional_traversal_tag
		, boost::iterator_facade::proxy_reference<std::pair<int, int>>
	  >

And do not downgrade iterator-category, if proxy_reference occurs.