feat(tazjin/rlox): Implement variable depth resolver

Implements the first part of the resolver from
https://craftinginterpreters.com/resolving-and-binding.html

This is wired up to the execution paths in main, but not yet in the
tests. The resolved depth is also not actually used for variable
lookups (yet).

Change-Id: I3a8615252b7b9b12d5a290c5ddf85988f61b9184
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2403
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/interpreter.rs

@@ -107,13 +107,13 @@ impl Environment {
     }
 
     fn get(&self, name: &parser::Variable) -> Result<Value, Error> {
-        let ident = identifier_str(&name.0)?;
+        let ident = identifier_str(&name.name)?;
 
         self.values
             .get(ident)
             .map(Clone::clone)
             .ok_or_else(|| Error {
-                line: name.0.line,
+                line: name.name.line,
                 kind: ErrorKind::UndefinedVariable(ident.into()),
             })
             .or_else(|err| {