Fix various things that were broken with USE_STD_STRING

In particular, this applies to the --vs-progress-string-statistics feature as
this requires the string_container to be available. Other than that, there are
also two places in the unit tests (which don't show up on travis because the
unit tests are currently not being built on Travis if USE_STD_STRING is set)

Author: martin

unit/util/sharing_map.cpp

@@ -492,7 +492,7 @@ TEST_CASE("Sharing map views and iteration", "[core][util]")
 
   SECTION("View")
   {
-    typedef std::pair<std::string, std::string> pt;
+    typedef std::pair<irep_idt, std::string> pt;
 
     sharing_map_standardt sm;
     sharing_map_standardt::viewt view;