refactor(tazjin/rlox): Prepare scanner for shared use

In the book, the clox interpreter has its own scanner which uses a
pull-based model for a single pass compiler.

I can't be bothered to write another scanner, or amend this one into
pull-mode to work with the treewalk interpreter, so instead I will
just reuse it and pull from a vector of tokens.

The tokens are shared between both interpreters and the scanner is not
what I'm interested in here.

Change-Id: Ib07e89127fce2b047f9b3e1ff7e9908d798b3b2b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2420
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/treewalk/mod.rs

@@ -1,5 +1,6 @@
+use crate::scanner;
+
 mod errors;
 pub mod interpreter;
 mod parser;
 mod resolver;
-mod scanner;