toml: disable the unused display Cargo feature (#59)

The `display` Cargo feature of the `toml` dependency is only used for
tests, so we disable it to reduce the effective size for users, and
enable it only for tests.
For reference, the default features of `toml` are `display` and `parse`.

Cargo.toml

@@ -18,11 +18,15 @@ path = "src/lib.rs"
 [dependencies]
 serde = { version = "1.0.114", features = ["derive"] }
 thiserror = "1.0.49"
-toml = { version = "0.8", features = ["preserve_order"] }
+toml = { version = "0.8", default-features = false, features = [
+  "parse",
+  "preserve_order",
+] }
 
 [dev-dependencies]
 insta = "1.39.0"
 tempfile = "3.10.1"
+toml = { version = "0.8", default-features = false, features = ["display"] }
 
 [package.metadata.release]
 pre-release-hook = ["git", "cliff", "-o", "--tag", "{{version}}"]