Remove unnecessary heap allocations in Rust protover implementation.

https://github.com/frewsxcv/tor/compare/frewsxcv-protover-heap?expand=1

This is my first Tor patch! 🎉

I was working towards https://trac.torproject.org/projects/tor/ticket/24030 and noticed we do some unnecessary heap allocations via collect.

Even though this is a small change I'm mostly opening to see what the review/merge process is like. Also curious if small patches like this should be folded into larger patches as separate commits, or if it's okay to open individual tickets for them. If there's anything I'm doing wrong, let me know – I've done virtually no open source work outside of GitHub and am new to this Trac-based workflow.

Trac:
Username: frewsxcv