Simplify code, document logic, refactor and better error messages.

src/scope.rs

@@ -85,6 +85,13 @@ impl Scope {
         &mut entry.1
     }
 
+    /// Get a mutable reference to a variable in the Scope and downcast it to a specific type
+    pub(crate) fn get_mut_by_type<T: Any + Clone>(&mut self, key: &str, index: usize) -> &mut T {
+        self.get_mut(key, index)
+            .downcast_mut::<T>()
+            .expect("wrong type cast")
+    }
+
     /// Get an iterator to variables in the Scope.
     pub fn iter(&self) -> impl Iterator<Item = (&str, &Dynamic)> {
         self.0