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/ruby/catches_strings_runme.rb

@@ -16,3 +16,16 @@ end
 if (!exception_thrown)
   raise RuntimeError, "Should have thrown an exception"
 end
+
+exception_thrown = false
+begin
+  Catches_strings::StringsThrower.stdstring()
+rescue RuntimeError => e
+  if (!e.to_s.include? "stdstring message")
+    raise RuntimeError, "incorrect exception message: #{e.to_s}"
+  end
+  exception_thrown = true
+end
+if (!exception_thrown)
+  raise RuntimeError, "Should have thrown an exception"
+end