Refactor HTTP identifier names (API Change):

fix #171

Several names and  HTTP identifiers are renamed to be
more consistent, self-explanatory, and concise:

* "Fields" is a collection of HTTP header fields (rfc7230 section 3.2)
* "Header" is the Start Line plus Fields. Another way to look at it is,
  the HTTP message minus the body.
* `basic_fields` replaces `basic_headers`
* `fields` replaces `headers`
* `Fields` replaces `Headers` in template parameter lists
* `header` replaces `message_headers`
* `header::fields` replaces `message_headers::fields`

The changes are cosmetic and do not affect run-time behavior.

test/http/parse_error.cpp

@@ -48,7 +48,7 @@ public:
         check("http", parse_error::invalid_chunk_size);
         check("http", parse_error::invalid_ext_name);
         check("http", parse_error::invalid_ext_val);
-        check("http", parse_error::headers_too_big);
+        check("http", parse_error::header_too_big);
         check("http", parse_error::body_too_big);
         check("http", parse_error::short_read);
     }