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/iteration_context.rs

@@ -3,6 +3,7 @@ use crate::renderer::context_element::ContextElement;
 use crate::renderer::Loopable;
 use crate::renderer::RenderError;
 use crate::renderer::Renderable;
+use crate::renderer::Truthiness;
 use crate::renderer::WalkError;
 use crate::{parser::Filter, parser::OwnedLiteral, renderer::Walkable};
 
@@ -30,6 +31,15 @@ impl IterationContext {
 
 impl ContextElement for IterationContext {}
 
+impl Truthiness for IterationContext {
+    fn is_truthy(&self) -> bool {
+        // TODO: Would this even ever be called? Won't matter, but I'd
+        // like to know. Since it is injected 1 above the current
+        // context, we wouldn't be able to access it with `{.}`.
+        true
+    }
+}
+
 impl Renderable for IterationContext {
     fn render(&self, _filters: &Vec<Filter>) -> Result<String, RenderError> {
         // TODO: Would this even ever be called? Won't matter, but I'd