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

src/lib.rs

@@ -21,6 +21,7 @@ mod lex;
 mod segment;
 mod stl;
 mod sym;
+mod error;
 
 pub mod ast {
     pub use crate::run::run;
@@ -28,6 +29,7 @@ pub mod ast {
     pub use crate::lex::lex;
     pub use crate::segment::{Ctr, Seg, Type};
     pub use crate::sym::{Args, SymTable, Symbol, UserFn, ValueType};
+    pub use crate::error::{Traceback, start_trace};
 }
 
 pub mod stdlib {