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/parser_bench.cpp

@@ -122,20 +122,20 @@ public:
             [&]
             {
                 testParser<nodejs_parser<
-                    true, streambuf_body, headers>>(
+                    true, streambuf_body, fields>>(
                         Repeat, creq_);
                 testParser<nodejs_parser<
-                    false, streambuf_body, headers>>(
+                    false, streambuf_body, fields>>(
                         Repeat, cres_);
             });
         timedTest(Trials, "http::basic_parser_v1",
             [&]
             {
                 testParser<parser_v1<
-                    true, streambuf_body, headers>>(
+                    true, streambuf_body, fields>>(
                         Repeat, creq_);
                 testParser<parser_v1<
-                    false, streambuf_body, headers>>(
+                    false, streambuf_body, fields>>(
                         Repeat, cres_);
             });
         pass();