Update indexmap requirement from 1.9.1 to 2.0.0

Updates the requirements on [indexmap](https://github.com/bluss/indexmap) to permit the latest version.
- [Changelog](https://github.com/bluss/indexmap/blob/master/RELEASES.md)
- [Commits](indexmap-rs/indexmap@1.9.1...2.0.0)

---
updated-dependencies:
- dependency-name: indexmap
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

crates/ir/Cargo.toml

@@ -20,4 +20,4 @@ smallvec = "1.7.0"
 fxhash = "0.2.1"
 dyn-clone = "1.0.4"
 sonatina-triple = { path = "../triple", version = "0.0.3-alpha" }
-indexmap = "1.9.1"
+indexmap = "2.0.0"