Help test out ThinLTO!

compiler
5

Tested crates.io:

cargo clean && RUSTFLAGS='-C codegen-units=16 -Zthinlto' cargo +nightly build --release: 175.77s

touch src/lib.rs && RUSTFLAGS='-C codegen-units=16 -Zthinlto' cargo +nightly build --release: 52.16 seconds

cargo clean && cargo +nightly build --release: 237.59s

touch src/lib.rs && cargo +nightly build --release: 119.96 seconds

So a full build including dependencies sped up 33%, and just rebuilding the main crate sped up 100%. Nice! (No benchmarks available on this one)