parser requires basic_fields (API Change):

* `parser` is now templated on Allocator

* `parser` only produces messages using `basic_fields<Allocator>`
  as the Fields type.

* message-oriented read and async_read only work on messages
  using basic_fields as the Fields type.

Actions Required:

* Callers using `parser` with Fields types other than basic_fields
  will need to create their own subclass of basic_parser to work
  with their custom fields type.

test/http/parser.cpp

@@ -32,7 +32,7 @@ class parser_test
 public:
     template<bool isRequest>
     using parser_type =
-        parser<isRequest, string_body, fields>;
+        parser<isRequest, string_body>;
 
     static
     boost::asio::const_buffers_1