Parser concept, fixes:

A new concept Parser is introduced with routines to read from a stream
into the parser. This solves a problem with the old read interface where
messages must be default constructible and move assignable.

Parser fixes:

* Fix detect invalid reason-phrase octets
* Fix write_eof to set the 'complete' state on success
* Fix consider parse complete if eof received on empty body

WebSocket:

* Increase coverage

test/core/buffer_cat.cpp

@@ -116,7 +116,8 @@ public:
 
         try
         {
-            expect((buffer_size(*bs.end()) == 0, false));
+            buffer_size(*bs.end());
+            fail();
         }
         catch(std::exception const&)
         {