code style improvements for src/rust/protover/ffi.rs
Was reading through `src/rust/protover/ffi.rs` and made a few changes along the way. Here's my git branch: https://github.com/frewsxcv/tor/compare/frewsxcv-stringlist-refactor Each change is isolated to its own commit. **Trac**: **Username**: frewsxcv
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