Speed: Use Rust libcst instead of LibCST?

I noticed parsing was slower on large projects. I believe it was the LibCST work that was the bottleneck.

- [ ] Consider [Rust libcst](https://crates.io/crates/libcst) as a drop-in replacement for LibCST?

Start by locating where LibCST is used to parse large projects and establish a baseline for the reported slowdown. Compare that behavior with Rust libcst as a possible drop-in replacement, checking compatibility and performance. Done means there is a clear decision about whether the replacement is viable, supported by benchmark results.

Written by the indexing model from the issue text.