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/go/catches_strings_runme.go

@@ -4,14 +4,29 @@ import "strings"
 import . "swigtests/catches_strings"
 
 func main() {
-  exception_thrown := false
-  func() {
-    defer func() {
-      exception_thrown = strings.Index(recover().(string), "charstring message") == 0
+  {
+    exception_thrown := false
+    func() {
+      defer func() {
+        exception_thrown = strings.Index(recover().(string), "charstring message") == 0
+      }()
+      StringsThrowerCharstring()
     }()
-    StringsThrowerCharstring()
-  }()
-  if !exception_thrown {
-    panic(0)
+    if !exception_thrown {
+      panic(0)
+    }
+  }
+
+  {
+    exception_thrown := false
+    func() {
+      defer func() {
+        exception_thrown = strings.Index(recover().(string), "stdstring message") == 0
+      }()
+      StringsThrowerStdstring()
+    }()
+    if !exception_thrown {
+      panic(0)
+    }
   }
 }