Makes flat_tree a proper container (#383)

Removes flat_tree::get_view()
Adds flat_tree{iterator, reverse_iterator, begin, end, rbegin, rend, data, operator[], at, front, back, size, empty}

close #362

doc/modules/ROOT/pages/index.adoc

@@ -133,7 +133,7 @@ auto receiver(std::shared_ptr<connection> conn) -> asio::awaitable<void>
 
       // The response must be consumed without suspending the
       // coroutine i.e. without the use of async operations.
-      for (auto const& elem : resp.value().get_view())
+      for (auto const& elem : resp.value())
          std::cout << elem.value << "\n";
 
       std::cout << std::endl;