Keep source code order for module items

Cargo.lock

@@ -1670,6 +1670,7 @@ dependencies = [
 name = "rustpython-derive"
 version = "0.1.2"
 dependencies = [
+ "indexmap",
  "maplit",
  "once_cell",
  "proc-macro2",

derive/Cargo.toml

@@ -20,3 +20,4 @@ rustpython-bytecode = { path = "../bytecode", version = "0.1.1" }
 maplit = "1.0"
 once_cell = "1.3.1"
 textwrap = "0.12.1"
+indexmap = "^1"

derive/src/util.rs

@@ -1,3 +1,4 @@
+use indexmap::map::IndexMap;
 use proc_macro2::{Span, TokenStream};
 use quote::{quote, ToTokens};
 use std::collections::HashMap;
@@ -18,7 +19,7 @@ pub(crate) const ALL_ALLOWED_NAMES: &[&str] = &[
 ];
 
 #[derive(Default)]
-pub(crate) struct ItemNursery(HashMap<(String, Vec<Attribute>), TokenStream>);
+pub(crate) struct ItemNursery(IndexMap<(String, Vec<Attribute>), TokenStream>);
 
 impl ItemNursery {
     pub fn add_item(