implement chapter 12 in rust

rust/rox/src/expression.rs

@@ -17,6 +17,18 @@ pub enum Expression {
         paren: Token,
         args: Vec<Expression>,
     },
+    Get {
+        object: Box<Expression>,
+        name: Token,
+    },
+    Set {
+        object: Box<Expression>,
+        name: Token,
+        value: Box<Expression>,
+    },
+    This {
+        keyword: Token,
+    },
     Grouping {
         expression: Box<Expression>,
     },