WebSocket refactoring and tests:

websocket:

* Move echo server to test/
* Fix warnings
* Fix maskgen being uncopyable
* Simplify utf8_checker special member declarations
* Fix stream move assignable when owning the next layer
* Add javadocs for stream special members
* Add stream unit tests
* Move throwing member definitions to the .ipp file
* Use get_lowest_layer in stream declaration
* Perform type checks at each call site instead of constructor
* Demote close_code to a non-class enum:
    Otherwise, application specific close codes
    cannot be assigned without using static_cast.

core:

* Add streambuf_readstream special members tests
* Add move assignment operator to streambuf_readstream
* Add detail/get_lowest_layer trait
* Add static_string tests
* Move static_string from websocket to core

test/streambuf_readstream.cpp

@@ -7,3 +7,40 @@
 
 // Test that header file is self-contained.
 #include <beast/streambuf_readstream.hpp>
+
+#include <beast/streambuf.hpp>
+#include <beast/detail/unit_test/suite.hpp>
+#include <boost/asio.hpp>
+
+namespace beast {
+
+class streambuf_readstream_test : public beast::detail::unit_test::suite
+{
+public:
+    void testSpecial()
+    {
+        using socket_type = boost::asio::ip::tcp::socket;
+        boost::asio::io_service ios;
+        {
+            streambuf_readstream<socket_type, streambuf> srs(ios);
+            streambuf_readstream<socket_type, streambuf> srs2(std::move(srs));
+            srs = std::move(srs2);
+        }
+        {
+            socket_type sock(ios);
+            streambuf_readstream<socket_type&, streambuf> srs(sock);
+            streambuf_readstream<socket_type&, streambuf> srs2(std::move(srs));
+        }
+        pass();
+    }
+
+    void run() override
+    {
+        testSpecial();
+    }
+};
+
+BEAST_DEFINE_TESTSUITE(streambuf_readstream,core,beast);
+
+} // beast
+