Remove lowest_layer, add get_lowest_layer (API Change):

fix #1417

* New get_lowest_layer free function works for any object
* New lowest_layer_type trait works for any object
* New examples and documentation on layered streams

API Changes:

* The member function lowest_layer is removed from all
  types provided by the library:

Actions Required:

* Call the free function get_lowest_layer instead of
  member lowest_layer.

test/asio.cpp

@@ -1 +1,15 @@
+//
+// Copyright (c) 2019 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/boostorg/beast
+//
+
+// This file is used to build the asio static library,
+// used with BOOST_ASIO_SEPARATE_COMPILATION which helps
+// reduce compilation time
+
 #include <boost/asio/impl/src.hpp>
+#include <boost/asio/ssl/impl/src.hpp>