Refactor extras:

A new directory extras/ is added, and code not part of the official
public Beast interface is moved there.

test/streambuf_readstream.cpp

@@ -9,12 +9,12 @@
 #include <beast/streambuf_readstream.hpp>
 
 #include <beast/streambuf.hpp>
-#include <beast/detail/unit_test/suite.hpp>
+#include <beast/unit_test/suite.hpp>
 #include <boost/asio.hpp>
 
 namespace beast {
 
-class streambuf_readstream_test : public beast::detail::unit_test::suite
+class streambuf_readstream_test : public beast::unit_test::suite
 {
 public:
     void testSpecial()