implement chapter 13 in rust!

rust/rox/src/native_functions.rs

@@ -8,7 +8,7 @@ use ordered_float::OrderedFloat;
 use crate::{
     callable::{Callable, CallingError},
     interpreter::Interpreter,
-    value::Value,
+    value::{CallableType, Value},
 };
 
 struct Clock;
@@ -37,5 +37,8 @@ impl Callable for Clock {
 }
 
 pub fn all() -> Vec<(String, Value)> {
-    vec![("clock".into(), Value::Callable(Rc::new(Clock {})))]
+    vec![(
+        "clock".into(),
+        Value::Callable((Rc::new(Clock {}), CallableType::Function)),
+    )]
 }