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

Jamroot

@@ -51,6 +51,7 @@ project beast
       <include>./include
       #<use>/boost//headers
       <library>/boost/system//boost_system
+      <library>/boost/coroutine//boost_coroutine
       <library>/boost/filesystem//boost_filesystem
       <library>/boost/program_options//boost_program_options
 #     <library>ssl