xtask: fix getters source order

Generate getters in the same order as the fields they access. Avoids git
noise and makes for better docs.

xtask/Cargo.toml

@@ -9,4 +9,5 @@ structopt = {version = "0.3", default-features = false }
 anyhow = "1"
 syn = {version = "1", features = ["visit-mut", "extra-traits"] }
 quote = "1"
-proc-macro2 = "1"
+proc-macro2 = "1"
+indexmap = "1.6"

xtask/src/codegen/getters.rs

@@ -1,5 +1,4 @@
-use std::collections::HashMap;
+use indexmap::IndexMap;
-
 use proc_macro2::{Span, TokenStream};
 use quote::{quote, TokenStreamExt};
 use syn::{
@@ -9,7 +8,7 @@ use syn::{
 
 pub struct GetterList<'a> {
     slf: Ident,
-    item_fields: HashMap<Ident, (&'a Item, &'a FieldsNamed)>,
+    item_fields: IndexMap<Ident, (&'a Item, &'a FieldsNamed)>,
 }
 
 impl<'a> GetterList<'a> {