add structural tests for partial strings, rename ast.rs

src/prolog/arithmetic.rs

@@ -1,6 +1,6 @@
 use prolog_parser::ast::*;
 
-use prolog::ast::*;
+use prolog::instructions::*;
 use std::cell::Cell;
 use std::cmp::{min, max};
 use std::rc::Rc;