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/lib.rs

@@ -9,6 +9,7 @@
 //!    [`enum@Error`]).
 
 #![warn(missing_docs)]
+extern crate self as argus_core;
 
 pub mod expr;
 pub mod prelude;