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/bin.rs

@@ -10,6 +10,7 @@ use renderer::DustRenderer;
 use renderer::Loopable;
 use renderer::RenderError;
 use renderer::Renderable;
+use renderer::Truthiness;
 use renderer::WalkError;
 use renderer::Walkable;
 use std::cmp::Ordering;
@@ -236,6 +237,19 @@ fn apply_filters(
 
 impl ContextElement for serde_json::Value {}
 
+impl Truthiness for serde_json::Value {
+    fn is_truthy(&self) -> bool {
+        match self {
+            serde_json::Value::Null => false,
+            serde_json::Value::Bool(boolean) => *boolean,
+            serde_json::Value::Number(_num) => true,
+            serde_json::Value::String(string_value) => !string_value.is_empty(),
+            serde_json::Value::Array(array_value) => !array_value.is_empty(),
+            serde_json::Value::Object(_obj) => true,
+        }
+    }
+}
+
 impl Renderable for serde_json::Value {
     fn render(&self, _filters: &Vec<Filter>) -> Result<String, RenderError> {
         let after_apply = if _filters.is_empty() {