refactor(tazjin/rlox): Remove use of sentinel values

The C code from which this is translated uses sentinel values for
various things, this commit replaces them with standard Rust types
instead (amongst a bunch of other small improvements).

Change-Id: I892811a7afebb5a0f3b825824fc493ab0b399e44
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3735
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/rlox/src/bytecode/tests.rs

@@ -138,12 +138,14 @@ fn local_variables() {
         r#"
           var a = 10;
           var b = 5;
-
+          var result = 0;
           {
             var b = 10;
             var c = 2;
-            a * b * c;
+            result = a * b * c;
           }
+
+          result;
         "#,
         200.0,
     );