Change Implementers fields from HashMap to IndexMap (#898)

This struct returned by `Schema::implementers_map` maps interface types
to (object or interface) types that implement that interface.
Now the ordering of implementers is deterministic
and matches their definition order in the schema.
This helps fuzzing, where the same entropy source
should generate the same test case.

crates/apollo-compiler/CHANGELOG.md

@@ -17,6 +17,21 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 ## Maintenance
 ## Documentation-->
 
+# [x.x.x] (unreleased) - 2024-mm-dd
+
+## BREAKING
+
+- **Change `Implementers` fields from `HashMap` to `IndexMap` - [SimonSapin] in [pull/898]**
+  This struct returned by `Schema::implementers_map` maps interface types
+  to (object or interface) types that implement that interface.
+  Now the ordering of implementers is deterministic
+  and matches their definition order in the schema.
+  This helps fuzzing, where the same entropy source should generate the same test case.
+
+[SimonSapin]: https://github.com/SimonSapin
+[pull/898]: https://github.com/apollographql/apollo-rs/pull/898
+
+
 # [1.0.0-beta.20](https://crates.io/crates/apollo-compiler/1.0.0-beta.20) - 2024-07-30
 
 ## Fixes