Remove summary feature (#823)

Remove the `summary` feature and make the `summary` module always build.
It's a small module, so it shouldn't hurt compile times, and it should
prevent accidental breakages that are only caught on CI.

justfile

@@ -64,7 +64,6 @@ changes:
 check: lint clippy test
 	git diff --no-ext-diff --quiet --exit-code
 	grep {{version}} CHANGELOG.md
-	cargo build --features summary
 	cargo +nightly generate-lockfile -Z minimal-versions
 	cargo test
 	git checkout Cargo.lock