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/errors.rs

@@ -1,4 +1,6 @@
+use crate::scanner::ScannerError;
 use crate::treewalk::interpreter::Value;
+
 use std::fmt;
 
 #[derive(Debug)]
@@ -39,3 +41,19 @@ impl fmt::Display for Error {
         write!(f, "[line {}] Error: {:?}", self.line, self.kind)
     }
 }
+
+impl From<ScannerError> for Error {
+    fn from(err: ScannerError) -> Self {
+        match err {
+            ScannerError::UnexpectedChar { line, unexpected } => Error {
+                line,
+                kind: ErrorKind::UnexpectedChar(unexpected),
+            },
+
+            ScannerError::UnterminatedString { line } => Error {
+                line,
+                kind: ErrorKind::UnterminatedString,
+            },
+        }
+    }
+}