feat(tazjin/rlox): Implement tree-walk interpreter of exprs

This is only a subset of the Lox spec so far. It implements the
language up to the runtime error chapter on
https://craftinginterpreters.com/evaluating-expressions.html

Change-Id: I295dbf4b6544420d6fe80b6aaba661fb21acdea6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2281
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/parser.rs

@@ -12,15 +12,15 @@ use crate::scanner::{Token, TokenKind};
 
 #[derive(Debug)]
 pub struct Binary<'a> {
-    left: Box<Expr<'a>>,
-    operator: Token<'a>,
-    right: Box<Expr<'a>>,
+    pub left: Box<Expr<'a>>,
+    pub operator: Token<'a>,
+    pub right: Box<Expr<'a>>,
 }
 
 #[derive(Debug)]
-pub struct Grouping<'a>(Box<Expr<'a>>);
+pub struct Grouping<'a>(pub Box<Expr<'a>>);
 
-#[derive(Debug)]
+#[derive(Debug, Clone, PartialEq)]
 pub enum Literal {
     Boolean(bool),
     Number(f64),
@@ -30,8 +30,8 @@ pub enum Literal {
 
 #[derive(Debug)]
 pub struct Unary<'a> {
-    operator: Token<'a>,
-    right: Box<Expr<'a>>,
+    pub operator: Token<'a>,
+    pub right: Box<Expr<'a>>,
 }
 
 #[derive(Debug)]