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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