re-include benches directory back to satisfy docs builder

`cargo publish --dry-run` complains that it can't parse the manifest
file. That's my best guess at the moment for why https://docs.rs isn't
posting documentation for this crate.

Cargo.toml

@@ -14,7 +14,6 @@ exclude = [
     ".github/",
     ".gitignore",
     ".npmignore",
-    "benches/",
     "data/",
     "examples/",
     "js/",