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/r/catches_strings_runme.R

@@ -13,3 +13,12 @@ tryCatch({
 }
 )
 unittest(exception_thrown, TRUE)
+
+exception_thrown = FALSE
+tryCatch({
+    StringsThrower_stdstring()
+}, error = function(e) {
+    exception_thrown <<- grepl(e$message, "stdstring message", fixed=TRUE)
+}
+)
+unittest(exception_thrown, TRUE)