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.

example/http/server/stackless-ssl/CMakeLists.txt

@@ -22,7 +22,7 @@ if (OPENSSL_FOUND)
     set_property(TARGET http-server-stackless-ssl PROPERTY FOLDER "example-http-server")
 
     target_link_libraries (http-server-stackless-ssl
-        ${OPENSSL_LIBRARIES}
+        OpenSSL::SSL OpenSSL::Crypto
         )
 
 endif()

example/http/server/stackless-ssl/Jamfile

@@ -7,14 +7,16 @@
 # Official repository: https://github.com/boostorg/beast
 #
 
+import ac ;
+
 project
     : requirements
-    <library>ssl
-    <library>crypto
+    [ ac.check-library /boost/beast//ssl : <library>/boost/beast//ssl : <build>no ]
+    <library>/boost/beast//crypto
     ;
 
-exe http-server-async-ssl :
-    http_server_async_ssl.cpp
+exe http-server-stackless-ssl :
+    http_server_stackless_ssl.cpp
     :
     <variant>coverage:<build>no
     <variant>ubasan:<build>no