Gluom and Profile-Guided Optimization (PGO)

Hi!

I think we can try to optimize Gluon tooling with Profile-Guided Optimization like it already does Rust [itself](https://blog.rust-lang.org/inside-rust/2020/11/11/exploring-pgo-for-the-rust-compiler.html). Since Gluon is written in Rust, [cargo-pgo](https://github.com/Kobzol/cargo-pgo) can help with it.