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

.gitignore

@@ -2,4 +2,6 @@
 target/
 **/*.rs.bk
 .DS_Store
-*.pdb
+*.pdb
+exercises/clippy/Cargo.toml
+exercises/clippy/Cargo.lock