Add a new trait to ContextElement for Truthiness.

Before I was relying on Loopable to both determine truthiness and get a list of elements to loop over. This will no longer work since I need to only set $idx and $len when iterating over actual arrays, as opposed to all truthy values, so I've finally made truthiness explicit.

src/renderer/context_element.rs

@@ -5,10 +5,20 @@ use std::any::Any;
 use std::{cmp::Ordering, fmt::Debug};
 
 pub trait ContextElement:
-    Debug + Walkable + Renderable + Loopable + CloneIntoBoxedContextElement + CompareContextElement
+    Debug
+    + Truthiness
+    + Walkable
+    + Renderable
+    + Loopable
+    + CloneIntoBoxedContextElement
+    + CompareContextElement
 {
 }
 
+pub trait Truthiness {
+    fn is_truthy(&self) -> bool;
+}
+
 pub trait Walkable {
     fn walk(&self, segment: &str) -> Result<&dyn ContextElement, WalkError>;
 }