Implement external functions

example.elna

@@ -119,6 +119,8 @@ begin
   writei(x)
 end;
 
+proc exit(code: Int); extern;
+
 begin
   test_primitive();
   test_string();
@@ -129,5 +131,7 @@ begin
   test_if();
   test_not();
   test_param(8, 7);
-  test_const_char()
+  test_const_char();
+
+  exit(0)
 end.