feat!(core): Use new AST structure

Derive Expr methods using a derive proc-macro. These macros are present in
the `argus-derive` crate, but the traits are defined in `argus-core`

argus-core/src/expr/iter.rs

@@ -47,7 +47,7 @@ impl<'a> Iterator for AstIter<'a> {
 mod tests {
     use itertools::Itertools;
 
-    use crate::expr::{Expr, ExprBuilder, ExprRef};
+    use crate::expr::{ExprBuilder, ExprRef};
 
     #[test]
     fn simple_iter() {