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_eval.rs

@@ -81,7 +81,8 @@ mod eval_tests {
         let doc_tree = lex(&test_doc).unwrap();
         match eval(&doc_tree, &mut syms) {
             Err(e) => {
-                assert_eq!(e, "error in call to undefined: undefined symbol: undefined")
+                assert_eq!(e.0.first().unwrap().message,
+                           "(is an undefined symbol)")
             }
 
             Ok(reduced) => {