feat: Add clippy lints

- adds a new 'clippy' category for exercises
- clippy exercises should throw no warnings
- install script now also installs clippy

is related to https://github.com/rust-lang/rust-clippy/issues/2604

src/run.rs

@@ -6,6 +6,7 @@ pub fn run(exercise: &Exercise) -> Result<(), ()> {
     match exercise.mode {
         Mode::Test => test(exercise)?,
         Mode::Compile => compile_and_run(exercise)?,
+        Mode::Clippy => compile_and_run(exercise)?,
     }
     Ok(())
 }