std::string throws typemaps

Go, Guile, Racket, Scilab: Add throws typemaps for std::string so that
thrown string exception messages can be seen.

Test all language for std::string throws typemaps

Examples/test-suite/catches_strings.i

@@ -3,11 +3,15 @@
 %include <std_string.i>
 
 %catches(const char *) StringsThrower::charstring;
+%catches(std::string) StringsThrower::stdstring;
 
 %inline %{
 struct StringsThrower {
   static void charstring() {
     throw "charstring message";
   }
+  static void stdstring() {
+    throw std::string("stdstring message");
+  }
 };
 %}