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/detail/invokable.hpp

@@ -85,8 +85,6 @@ public:
 #endif
 
     invokable() = default;
-    invokable(invokable const&) = delete;
-    invokable& operator=(invokable const&) = delete;
 
     invokable(invokable&& other)
     {