Refactor map code

* `Map` and `InnerMap` are now fully immutable
* Internal `Deref` impl has been removed to pave for the future
  introduction of a map trait
* Iterators have been reworked

Author: marmeladema

engine/src/ast/function_expr.rs

@@ -295,7 +295,7 @@ impl ValueExpr for FunctionCallExpr {
                 // Extract the values of the map
                 if let LhsValue::Map(map) = first {
                     first = LhsValue::Array(
-                        Array::try_from_iter(map.value_type(), map.values_into_iter()).unwrap(),
+                        Array::try_from_iter(map.value_type(), map.into_values()).unwrap(),
                     );
                 }
                 // Retrieve the underlying `Array`