feat: Refactor hint system

Hints are now accessible using the CLI subcommand `rustlings hint
<exercise name`.

BREAKING CHANGE: This fundamentally changes the way people interact with exercises.

exercises/variables/variables4.rs

@@ -1,45 +1,7 @@
 // variables4.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute the command `rustlings hint variables4` if you want a hint :)
 
 fn main() {
     let x: i32;
     println!("Number {}", x);
 }
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-// Oops! In this exercise, we have a variable binding that we've created on
-// line 5, and we're trying to use it on line 6, but we haven't given it a
-// value. We can't print out something that isn't there; try giving x a value!
-// This is an error that can cause bugs that's very easy to make in any
-// programming language -- thankfully the Rust compiler has caught this for us!