apply automatic clippy fixes to examples

[coriolinus]

for ex in exercises/*; do
   (
     cd "$ex"
     cp example.rs src/lib.rs
     if [ -f Cargo-example.toml ]; then
       cp Cargo-example.toml Cargo.toml
     fi
     cargo +nightly clippy -Z unstable-options --fix --allow-dirty
     cp src/lib.rs example.rs
     git checkout -- src/lib.rs Cargo.toml
  )
done

Helps address #1012.