Reorder some includes
Summary:
This makes a later diff slightly cleaner.  Plus, standard headers
should be above non-standard.
#include <mutex>
#include <stdexcept>
#include <unordered_map>
-#include <cxxabi.h>
-#include <unwind.h>
#include <cassert>
+#include <unwind.h>
+#include <cxxabi.h>
#include <lyra/lyra_exceptions.h>
