net::ssl::stream is canonical in examples

We no longer need to use beast::ssl_stream to improve write performance, as
Asio now linearizes scatter/gather I/O in ssl::stream:
https://github.com/chriskohlhoff/asio/commit/17637a48ccbfa2f63941d8393a7c8316a8df4a79

example/http/client/async-ssl-system-executor/http_client_async_ssl_system_executor.cpp

@@ -17,15 +17,13 @@
 
 #include <boost/beast/core.hpp>
 #include <boost/beast/http.hpp>
-#include <boost/beast/ssl.hpp>
 #include <boost/beast/version.hpp>
+#include <boost/asio/ssl.hpp>
 #include <boost/asio/strand.hpp>
 #include <boost/asio/system_executor.hpp>
 #include <cstdlib>
-#include <functional>
 #include <iostream>
 #include <memory>
-#include <string>
 
 namespace beast = boost::beast;         // from <boost/beast.hpp>
 namespace http = beast::http;           // from <boost/beast/http.hpp>
@@ -46,7 +44,7 @@ fail(beast::error_code ec, char const* what)
 class session : public std::enable_shared_from_this<session>
 {
     tcp::resolver resolver_;
-    beast::ssl_stream<beast::tcp_stream> stream_;
+    ssl::stream<beast::tcp_stream> stream_;
     beast::flat_buffer buffer_; // (Must persist between reads)
     http::request<http::empty_body> req_;
     http::response<http::string_body> res_;

example/http/client/async-ssl/http_client_async_ssl.cpp

@@ -17,14 +17,12 @@
 
 #include <boost/beast/core.hpp>
 #include <boost/beast/http.hpp>
-#include <boost/beast/ssl.hpp>
 #include <boost/beast/version.hpp>
+#include <boost/asio/ssl.hpp>
 #include <boost/asio/strand.hpp>
 #include <cstdlib>
-#include <functional>
 #include <iostream>
 #include <memory>
-#include <string>
 
 namespace beast = boost::beast;         // from <boost/beast.hpp>
 namespace http = beast::http;           // from <boost/beast/http.hpp>
@@ -45,7 +43,7 @@ fail(beast::error_code ec, char const* what)
 class session : public std::enable_shared_from_this<session>
 {
     tcp::resolver resolver_;
-    beast::ssl_stream<beast::tcp_stream> stream_;
+    ssl::stream<beast::tcp_stream> stream_;
     beast::flat_buffer buffer_; // (Must persist between reads)
     http::request<http::empty_body> req_;
     http::response<http::string_body> res_;

example/http/client/awaitable-ssl/http_client_awaitable_ssl.cpp

@@ -15,18 +15,17 @@
 
 #include <boost/beast/core.hpp>
 #include <boost/beast/http.hpp>
-#include <boost/beast/ssl.hpp>
 #include <boost/beast/version.hpp>
 #include <boost/asio/as_tuple.hpp>
 #include <boost/asio/awaitable.hpp>
 #include <boost/asio/co_spawn.hpp>
 #include <boost/asio/detached.hpp>
+#include <boost/asio/ssl.hpp>
 #include <boost/asio/use_awaitable.hpp>
 
 #if defined(BOOST_ASIO_HAS_CO_AWAIT)
 
 #include <cstdlib>
-#include <functional>
 #include <iostream>
 #include <string>
 #include "example/common/root_certificates.hpp"
@@ -58,7 +57,7 @@ do_session(
     using tcp_stream = typename beast::tcp_stream::rebind_executor<executor_with_default>::other;
 
     // We construct the ssl stream from the already rebound tcp_stream.
-    beast::ssl_stream<tcp_stream> stream{
+    ssl::stream<tcp_stream> stream{
         net::use_awaitable.as_default_on(beast::tcp_stream(co_await net::this_coro::executor)),
         ctx};
 

example/http/client/coro-ssl/http_client_coro_ssl.cpp

@@ -17,9 +17,9 @@
 
 #include <boost/beast/core.hpp>
 #include <boost/beast/http.hpp>
-#include <boost/beast/ssl.hpp>
 #include <boost/beast/version.hpp>
 #include <boost/asio/spawn.hpp>
+#include <boost/asio/ssl.hpp>
 #include <cstdlib>
 #include <functional>
 #include <iostream>
@@ -55,7 +55,7 @@ do_session(
 
     // These objects perform our I/O
     tcp::resolver resolver(ioc);
-    beast::ssl_stream<beast::tcp_stream> stream(ioc, ctx);
+    ssl::stream<beast::tcp_stream> stream(ioc, ctx);
 
     // Set SNI Hostname (many hosts need this to handshake successfully)
     if(! SSL_set_tlsext_host_name(stream.native_handle(), host.c_str()))

example/http/client/sync-ssl/http_client_sync_ssl.cpp

@@ -17,15 +17,12 @@
 
 #include <boost/beast/core.hpp>
 #include <boost/beast/http.hpp>
-#include <boost/beast/ssl.hpp>
 #include <boost/beast/version.hpp>
 #include <boost/asio/connect.hpp>
 #include <boost/asio/ip/tcp.hpp>
-#include <boost/asio/ssl/error.hpp>
-#include <boost/asio/ssl/stream.hpp>
+#include <boost/asio/ssl.hpp>
 #include <cstdlib>
 #include <iostream>
-#include <string>
 
 namespace beast = boost::beast; // from <boost/beast.hpp>
 namespace http = beast::http;   // from <boost/beast/http.hpp>
@@ -67,7 +64,7 @@ int main(int argc, char** argv)
 
             // These objects perform our I/O
         tcp::resolver resolver(ioc);
-        beast::ssl_stream<beast::tcp_stream> stream(ioc, ctx);
+        ssl::stream<beast::tcp_stream> stream(ioc, ctx);
 
         // Set SNI Hostname (many hosts need this to handshake successfully)
         if(! SSL_set_tlsext_host_name(stream.native_handle(), host))