[Feature request] Consider adding support of reference proxy

Continuing https://github.com/boostorg/iterator/issues/47.
Currently, `iterator_facade` does not allow to have RandomAcess iterator with reference proxy (aka `std::vector::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
, boost::bidirectional_traversal_tag
, boost::iterator_facade::proxy_reference>
>
```
And do not downgrade iterator-category, if proxy_reference occurs.

Lesen Sie die im Issue verlinkte Dokumentation zu iterator_facade und prüfen Sie die Fortsetzung aus Issue 47, um die Regeln für die Ableitung von iterator-category zu verstehen. Definieren Sie, was die Unterstützung von RandomAccess-Iteratoren mit Proxy-Referenzen bedeuten soll, einschließlich der Frage, ob ein Proxy-Tag erforderlich ist, und verifizieren Sie, dass die resultierende API kein manuelles Überschreiben von iterator_category mehr erfordert.

Vom Indexierungsmodell aus dem Issue-Text verfasst.