switch from boost::{shared,weak}_ptr to std::{shared,weak}_ptr

This is mostly a simple lexical search+replace but the absence of operator< for
std::weak_ptr<T> leads to some complications, particularly with Evoral::Sequence
and ExportPortChannel.

gtk2_ardour/loudness_settings.cc

@@ -16,6 +16,8 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#include <cassert>
+
 #include <glibmm.h>
 #include "pbd/gstdio_compat.h"