Error Messaging Redesign

This commit contains the following:

* New data types to support full tracebacks
* New traceback data type used across stl and ast
* Updates to tests
* fixes for error messaging in sym and some stl functions

tests/test_lib_control.rs

@@ -233,8 +233,8 @@ mod control_lib_tests {
 
         eval(&doc_tree, &mut syms).unwrap();
         assert_eq!(
-            eval(&test_tree, &mut syms).err().unwrap(),
-            "error in call to result: undefined symbol: result".to_string()
+            eval(&test_tree, &mut syms).err().unwrap().0.first().unwrap().message,
+            "(is an undefined symbol)".to_string()
         );
     }
 }