Help test out ThinLTO!

compiler
28

@alexcrichton

$ rustc +nightly --version
rustc 1.22.0-nightly (692b94ae2 2017-10-09)
$ RUSTFLAGS='-C codegen-units=1' cargo +nightly build
      128.87 real       369.70 user        30.71 sys
$ RUSTFLAGS='-C codegen-units=16' cargo +nightly build
      118.54 real       477.28 user        38.48 sys
$ RUSTFLAGS='-C codegen-units=1' cargo +nightly build --release
      364.40 real       911.12 user        31.99 sys
$ RUSTFLAGS='-C codegen-units=16 -Z thinlto' cargo +nightly build --release
      247.08 real      1296.57 user        41.38 sys
$ RUSTFLAGS='-C codegen-units=16 -Z thinlto -Z inline-in-all-cgus=no' cargo +nightly build --release
      280.06 real      1427.57 user        48.12 sys