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/schemerunme/catches_strings.scm

@@ -2,4 +2,8 @@
               (StringsThrower-charstring))
 ; TODO: check the exception message
 
+(expect-throw 'swig-exception
+              (StringsThrower-stdstring))
+; TODO: check the exception message
+
 (exit 0)