Help test out ThinLTO!

compiler
15

Compile times for an internal work project:

$ RUSTFLAGS='-C codegen-units=1' cargo +nightly build
      138.79 real       400.32 user        33.90 sys
$ RUSTFLAGS='-C codegen-units=16' cargo +nightly build
      119.96 real       506.58 user        41.06 sys
$ RUSTFLAGS='-C codegen-units=1' cargo +nightly build --release
      370.87 real       934.23 user        32.40 sys
$ RUSTFLAGS='-C codegen-units=16 -Z thinlto' cargo +nightly build --release
      284.49 real      1357.25 user        47.25 sys