Phase 9.75-B: Convert StreamBox from Arc<Mutex> to RwLock, progress on DebugBox

Co-authored-by: moe-charm <217100418+moe-charm@users.noreply.github.com>

src/box_trait.rs

@@ -176,7 +176,7 @@ impl StringBox {
     /// Join array elements using this string as delimiter
     pub fn join(&self, array_box: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         if let Some(array) = array_box.as_any().downcast_ref::<ArrayBox>() {
-            let strings: Vec<String> = array.items.lock().unwrap()
+            let strings: Vec<String> = array.items.read().unwrap()
                 .iter()
                 .map(|element| element.to_string_box().value)
                 .collect();