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/parameters_context.rs

@@ -6,6 +6,7 @@ use crate::renderer::walking::walk_path;
 use crate::renderer::Loopable;
 use crate::renderer::RenderError;
 use crate::renderer::Renderable;
+use crate::renderer::Truthiness;
 use crate::renderer::WalkError;
 use crate::renderer::Walkable;
 use std::{cmp::Ordering, collections::HashMap};
@@ -67,6 +68,15 @@ impl ParametersContext {
 
 impl ContextElement for ParametersContext {}
 
+impl Truthiness for ParametersContext {
+    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 ParametersContext {
     fn render(&self, _filters: &Vec<Filter>) -> Result<String, RenderError> {
         // TODO: Would this even ever be called? Won't matter, but I'd
@@ -128,6 +138,15 @@ impl CompareContextElement for ParametersContext {
 
 impl ContextElement for OwnedLiteral {}
 
+impl Truthiness for OwnedLiteral {
+    fn is_truthy(&self) -> bool {
+        match self {
+            OwnedLiteral::LString(text) => !text.is_empty(),
+            OwnedLiteral::LPositiveInteger(num) => true,
+        }
+    }
+}
+
 impl Renderable for OwnedLiteral {
     fn render(&self, _filters: &Vec<Filter>) -> Result<String, RenderError> {
         match self {