feat(tazjin/rlox): Bootstrap recursive-descent parser for Lox

... mostly some AST boilerplate and a first top-level rule, plus
boilerplate similar to that set up in the Scanner.

Change-Id: I605d1de23c47a3b3702ab4f62cd3371bc3988c7d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2194
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/main.rs

@@ -6,6 +6,7 @@ use std::process;
 
 mod errors;
 mod interpreter;
+mod parser;
 mod scanner;
 
 fn main() {