More polishing

Changed

• Report inconsistent async more accurately (#26) @SfietKonstantin
• Forbid unsafe (#24) @SfietKonstantin
• Style: worked-around rustfmt (#23) @SfietKonstantin
• Cleanups (#22) @SfietKonstantin
• Derive some useful traits (#28) @orzogc

Fixed

• remove Cargo.lock and add Cargo.lock in .gitignore (#21) @orzogc