Warn against select few more rustc lints

The first one is enabled in the `rustc` itself and vendored-in tools:
`unused_lifetimes`. It makes sense to warn against it by default and
pepper allows or change the code such that it's used, e.g. with
PhantomData.

The second one will be already uplifted to `dead_code` (which is
warn-by-default) but since we're warning against some lints, it can't
hurt.

.cargo/config.toml

@@ -11,6 +11,8 @@ lto = true
 rustflags = [
 	# rustc additional warnings:
 	"-Wunused_crate_dependencies",
+	"-Wunused_lifetimes",
+	"-Wunused_tuple_struct_fields", # Will be uplifed into `dead_code` (warn-by-default) with https://github.com/rust-lang/rust/pull/118297
 	# Rust 2018 idioms that are not yet warn-by-default:
 	"-Welided_lifetimes_in_paths",
 	"-Wunused_extern_crates",