modernize std::string("foo") into "foo"s

* spot/ltsmin/ltsmin.cc, spot/misc/tmpfile.cc,
spot/parseaut/parseaut.yy, spot/taalgos/dot.cc, spot/tl/hierarchy.cc,
spot/tl/unabbrev.cc, spot/twa/acc.cc, spot/twa/twagraph.cc,
spot/twaalgos/dot.cc, spot/twaalgos/hoa.cc, spot/twaalgos/lbtt.cc,
spot/twaalgos/neverclaim.cc, spot/twaalgos/strength.cc,
spot/twaalgos/word.cc: Replace std::string("foo") by "foo"s, and
include namespace std::string_literals.

spot/tl/hierarchy.cc

@@ -32,6 +32,7 @@
 #include <spot/twaalgos/totgba.hh>
 #include <spot/twaalgos/cobuchi.hh>
 
+using namespace std::string_literals;
 
 namespace spot
 {
@@ -460,7 +461,7 @@ namespace spot
           goto break2;
         default:
           throw std::runtime_error
-            (std::string("nesting_depth(): unknown operator '") + c + '\'');
+            ("nesting_depth(): unknown operator '"s + c + '\'');
         }
   break2:
     if (want_nnf)

spot/tl/unabbrev.cc

@@ -21,6 +21,8 @@
 #include "config.h"
 #include <spot/tl/unabbrev.hh>
 
+using namespace std::string_literals;
+
 namespace spot
 {
   unabbreviator::unabbreviator(const char* opt)
@@ -61,9 +63,7 @@ namespace spot
           re_some_bool_ = true;
           break;
         default:
-          throw std::runtime_error
-            (std::string("unknown unabbreviation option: ")
-             + c);
+          throw std::runtime_error("unknown unabbreviation option: "s + c);
         }
   }