Remove unused and obsolete classes and tidy up:

Many classes required to support type-erasure of handlers and boost::asio
types are now obsolete, so these classes and files are removed:
HTTPClientType, FixedInputBuffer, PeerRole, socket_wrapper,
client_session, basic_url, abstract_socket, buffer_sequence, memory_buffer,
enable_wait_for_async, shared_handler, wrap_handler, streambuf,
ContentBodyBuffer, SSLContext, completion-handler based handshake detectors.
These structural changes are made:
* Some missing includes added to headers
* asio module directory flattened

beast/module/core/diagnostic/FatalError.cpp

@@ -17,6 +17,7 @@
 */
 //==============================================================================
 
+#include <beast/module/core/diagnostic/FatalError.h>
 #include <beast/unit_test/suite.h>
 
 namespace beast {