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/renderer.rs

@@ -524,12 +524,19 @@ mod tests {
     use crate::parser::Filter;
     use crate::renderer::context_element::Loopable;
     use crate::renderer::context_element::Renderable;
+    use crate::renderer::context_element::Truthiness;
     use crate::renderer::context_element::Walkable;
     use crate::renderer::CompareContextElement;
     use std::cmp::Ordering;
 
     impl ContextElement for String {}
 
+    impl Truthiness for String {
+        fn is_truthy(&self) -> bool {
+            !self.is_empty()
+        }
+    }
+
     impl Renderable for String {
         fn render(&self, _filters: &Vec<Filter>) -> Result<String, RenderError> {
             Ok(self.clone())
@@ -569,6 +576,12 @@ mod tests {
     }
     impl ContextElement for u64 {}
 
+    impl Truthiness for u64 {
+        fn is_truthy(&self) -> bool {
+            true
+        }
+    }
+
     impl Renderable for u64 {
         fn render(&self, _filters: &Vec<Filter>) -> Result<String, RenderError> {
             Ok(self.to_string())
@@ -605,6 +618,12 @@ mod tests {
 
     impl<I: 'static + ContextElement + Clone> ContextElement for HashMap<String, I> {}
 
+    impl<I: ContextElement> Truthiness for HashMap<String, I> {
+        fn is_truthy(&self) -> bool {
+            true
+        }
+    }
+
     impl<I: ContextElement> Renderable for HashMap<String, I> {
         fn render(&self, _filters: &Vec<Filter>) -> Result<String, RenderError> {
             // TODO: handle the filters