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/advanced/Jamfile

@@ -9,5 +9,5 @@
 
 build-project server ;
 
-# VFALCO How do I make this work on Windows and if OpenSSL is not available?
-#build-project server-flex ;
+# SSL
+build-project server-flex ;

example/advanced/server-flex/CMakeLists.txt

@@ -23,7 +23,7 @@ if (OPENSSL_FOUND)
     set_property(TARGET advanced-server-flex PROPERTY FOLDER "example-advanced-server")
     
     target_link_libraries (advanced-server-flex
-        ${OPENSSL_LIBRARIES}
+        OpenSSL::SSL OpenSSL::Crypto
         )
 
 endif()

example/advanced/server-flex/Jamfile

@@ -7,10 +7,12 @@
 # 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 advanced-server-flex :