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

exercises/clippy/clippy1.rs

@@ -0,0 +1,15 @@
+// clippy1.rs
+// The Clippy tool is a collection of lints to analyze your code
+// so you can catch common mistakes and improve your Rust code.
+//
+// Execute `rustlings hint clippy1` for hints :)
+
+// I AM NOT DONE
+
+fn main() {
+    let x = 1.2331f64;
+    let y = 1.2332f64;
+    if y != x {
+        println!("Success!");
+    }
+}