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

examples/http_server.cpp

@@ -19,7 +19,7 @@
 
 #include "http_async_server.hpp"
 #include "http_sync_server.hpp"
-#include "sig_wait.hpp"
+#include "../test/sig_wait.hpp"
 
 #include <boost/program_options.hpp>
 
@@ -69,13 +69,8 @@ int main(int ac, char const* av[])
     endpoint_type ep{address_type::from_string(ip), port};
 
     if(sync)
-    {
         http_sync_server server(ep, root);
-        sig_wait();
-    }
     else
-    {
         http_async_server server(ep, threads, root);
-        sig_wait();
-    }
+    sig_wait();
 }