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/mod.rs

@@ -13,6 +13,7 @@ pub use context_element::CompareContextElement;
 pub use context_element::ContextElement;
 pub use context_element::Loopable;
 pub use context_element::Renderable;
+pub use context_element::Truthiness;
 pub use context_element::Walkable;
 pub use errors::CompileError;
 pub use errors::RenderError;