beast support per-op cancellation

- websocket supports cancellation.
 - Iterating test for ws cancellation.
 - Only terminal cancellation is forwarded by default.
 - basic_stream supports cancellation.
 - supported cancellation is documented.
 - http cancellation additions.
 - Added cancellation_slot tests to http, utils and saved_handler.
 - Added post to write.cpp, to avoid SIGSEV in test.
 - Refresher describes cancellation in more detail.

include/boost/beast/http/write.hpp

@@ -161,6 +161,18 @@ write_some(
     this function. Invocation of the handler will be performed in a
     manner equivalent to using `net::post`.
 
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
     @see serializer
 */
 template<
@@ -291,6 +303,18 @@ write_header(
     @note The implementation will call @ref serializer::split with
     the value `true` on the serializer passed in.
 
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
     @see serializer
 */
 template<
@@ -412,6 +436,18 @@ write(
     this function. Invocation of the handler will be performed in a
     manner equivalent to using `net::post`.
 
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
     @see serializer
 */
 template<
@@ -637,6 +673,18 @@ write(
     this function. Invocation of the handler will be performed in a
     manner equivalent to using `net::post`.
 
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
     @see message
 */
 template<
@@ -699,6 +747,18 @@ async_write(
     this function. Invocation of the handler will be performed in a
     manner equivalent to using `net::post`.
 
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
     @see message
 */
 template<