Replace uses of net::coroutine with asio::coroutine

`asio::coroutine` is an extension, which is not part of
the Networking TS, so the `net` alias is not appropriate in this case.

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

example/echo-op/echo_op.cpp

@@ -41,7 +41,7 @@ async_echo (AsyncStream& stream, DynamicBuffer& buffer, CompletionToken&& token)
     This function is used to asynchronously read a line ending
     in a newline (`"\n"`) from the stream, and then write
     it back.
-    
+
     This call always returns immediately. The asynchronous operation
     will continue until one of the following conditions is true:
 
@@ -76,7 +76,7 @@ async_echo (AsyncStream& stream, DynamicBuffer& buffer, CompletionToken&& token)
         beast::error_code error      // Result of operation.
     );
     @endcode
-    
+
     Regardless of whether the asynchronous operation completes immediately or
     not, the handler will not be invoked from within this function. Invocation
     of the handler will be performed in a manner equivalent to using
@@ -161,10 +161,10 @@ async_echo(
 
     // This nested class implements the echo composed operation as a
     // stateful completion handler. We derive from `async_base` to
-    // take care of boilerplate and we derived from net::coroutine to
+    // take care of boilerplate and we derived from asio::coroutine to
     // allow the reenter and yield keywords to work.
 
-    struct echo_op : base_type, net::coroutine
+    struct echo_op : base_type, boost::asio::coroutine
     {
         AsyncStream& stream_;
         DynamicBuffer& buffer_;

example/http/server/stackless-ssl/http_server_stackless_ssl.cpp

@@ -238,7 +238,7 @@ fail(beast::error_code ec, char const* what)
 
 // Handles an HTTP server connection
 class session
-    : public net::coroutine
+    : public boost::asio::coroutine
     , public std::enable_shared_from_this<session>
 {
     // This is the C++11 equivalent of a generic lambda.
@@ -398,7 +398,7 @@ public:
 
 // Accepts incoming connections and launches the sessions
 class listener
-    : public net::coroutine
+    : public boost::asio::coroutine
     , public std::enable_shared_from_this<listener>
 {
     net::io_context& ioc_;

example/http/server/stackless/http_server_stackless.cpp

@@ -214,7 +214,7 @@ fail(beast::error_code ec, char const* what)
 
 // Handles an HTTP server connection
 class session
-    : public net::coroutine
+    : public boost::asio::coroutine
     , public std::enable_shared_from_this<session>
 {
     // This is the C++11 equivalent of a generic lambda.
@@ -344,7 +344,7 @@ public:
 
 // Accepts incoming connections and launches the sessions
 class listener
-    : public net::coroutine
+    : public boost::asio::coroutine
     , public std::enable_shared_from_this<listener>
 {
     net::io_context& ioc_;
@@ -407,7 +407,7 @@ public:
 private:
 
     #include <boost/asio/yield.hpp>
-    
+
     void
     loop(beast::error_code ec = {})
     {

example/websocket/server/stackless-ssl/websocket_server_stackless_ssl.cpp

@@ -48,7 +48,7 @@ fail(beast::error_code ec, char const* what)
 
 // Echoes back all received WebSocket messages
 class session
-    : public net::coroutine
+    : public boost::asio::coroutine
     , public std::enable_shared_from_this<session>
 {
     websocket::stream<beast::ssl_stream<beast::tcp_stream>> ws_;
@@ -164,7 +164,7 @@ public:
 
 // Accepts incoming connections and launches the sessions
 class listener
-    : public net::coroutine
+    : public boost::asio::coroutine
     , public std::enable_shared_from_this<listener>
 {
     net::io_context& ioc_;

example/websocket/server/stackless/websocket_server_stackless.cpp

@@ -43,7 +43,7 @@ fail(beast::error_code ec, char const* what)
 
 // Echoes back all received WebSocket messages
 class session
-    : public net::coroutine
+    : public boost::asio::coroutine
     , public std::enable_shared_from_this<session>
 {
     websocket::stream<beast::tcp_stream> ws_;
@@ -65,7 +65,7 @@ public:
     }
 
     #include <boost/asio/yield.hpp>
-    
+
     void
     loop(
         beast::error_code ec,
@@ -141,7 +141,7 @@ public:
 
 // Accepts incoming connections and launches the sessions
 class listener
-    : public net::coroutine
+    : public boost::asio::coroutine
     , public std::enable_shared_from_this<listener>
 {
     net::io_context& ioc_;
@@ -202,7 +202,7 @@ public:
 private:
 
     #include <boost/asio/yield.hpp>
-    
+
     void
     loop(beast::error_code ec = {})
     {