Make IndexMap have explicit serde support

Cargo.toml

@@ -16,7 +16,7 @@ license = "MIT"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-indexmap = "2.6.0"
+indexmap = { version = "2.6.0", features = ["serde"] }
 thiserror = "1.0"
 rand = "0.8"
 serde = { version = "1.0", features = ["derive"], optional = true }