feat(tazjin/rlox): Implement unary negation operator

Change-Id: I9a5bd3581d4ed05371651697ec496341eb7971ae
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2572
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

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

@@ -4,3 +4,13 @@ pub enum Value {
     Bool(bool),
     Number(f64),
 }
+
+impl Value {
+    pub fn is_falsey(&self) -> bool {
+        match self {
+            Value::Nil => true,
+            Value::Bool(false) => true,
+            _ => false,
+        }
+    }
+}