Add tests and a Sizable trait for ContextElement.

src/bin.rs

@@ -15,6 +15,7 @@ use renderer::Loopable;
 use renderer::MathNumber;
 use renderer::RenderError;
 use renderer::Renderable;
+use renderer::Sizable;
 use renderer::Truthiness;
 use renderer::WalkError;
 use renderer::Walkable;
@@ -313,6 +314,12 @@ impl Loopable for serde_json::Value {
     }
 }
 
+impl Sizable for serde_json::Value {
+    fn get_size<'a>(&'a self) -> Option<IceResult<'a>> {
+        todo!()
+    }
+}
+
 impl Castable for serde_json::Value {
     fn cast_to_type<'a>(&'a self, target: &str) -> Option<IceResult<'a>> {
         match (self, target) {