Refactor message and message_headers declarations:

message_headers is now a set of partial class template
specializations instead of a template class alias. This solves
a problem where template functions taking message_headers as a
parameter could not deduce argument types, since std::conditional
obscured the deduced context.

Both classes are refactored to share declarations using an #ifdef,
to eliminate an ugly set of extra declarations needed when building
the documentation.

Copy and move class special members are added.

A new function message::base() is provided which returns the
message_headers portion of a message.

include/beast/http/impl/message.ipp

@@ -20,6 +20,43 @@
 namespace beast {
 namespace http {
 
+template<class Headers>
+void
+swap(
+    message_headers<true, Headers>& m1,
+    message_headers<true, Headers>& m2)
+{
+    using std::swap;
+    swap(m1.version, m2.version);
+    swap(m1.method, m2.method);
+    swap(m1.url, m2.url);
+    swap(m1.headers, m2.headers);
+}
+
+template<class Headers>
+void
+swap(
+    message_headers<false, Headers>& a,
+    message_headers<false, Headers>& b)
+{
+    using std::swap;
+    swap(a.version, b.version);
+    swap(a.status, b.status);
+    swap(a.reason, b.reason);
+    swap(a.headers, b.headers);
+}
+
+template<bool isRequest, class Body, class Headers>
+void
+swap(
+    message<isRequest, Body, Headers>& m1,
+    message<isRequest, Body, Headers>& m2)
+{
+    using std::swap;
+    swap(m1.base(), m2.base());
+    swap(m1.body, m2.body);
+}
+
 template<bool isRequest, class Body, class Headers>
 bool
 is_keep_alive(message<isRequest, Body, Headers> const& msg)