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

include/beast/websocket/impl/write_frame_op.ipp

@@ -61,7 +61,8 @@ class stream<NextLayer>::write_frame_op
             fh.rsv2 = 0;
             fh.rsv3 = 0;
             fh.len = boost::asio::buffer_size(cb);
-            if((fh.mask = (ws.role_ == role_type::client)))
+            fh.mask = ws.role_ == role_type::client;
+            if(fh.mask)
             {
                 fh.key = ws.maskgen_();
                 detail::prepare_key(key, fh.key);