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/interpreter.rs

@@ -200,7 +200,9 @@ impl Lox for Interpreter {
     fn interpret(&mut self, code: String) -> Result<Value, Vec<Error>> {
         let chars: Vec<char> = code.chars().collect();
 
-        let mut program = scanner::scan(&chars).and_then(|tokens| parser::parse(tokens))?;
+        let mut program = scanner::scan(&chars)
+            .map_err(|errors| errors.into_iter().map(Into::into).collect())
+            .and_then(|tokens| parser::parse(tokens))?;
 
         let globals = self
             .env