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/variables3.rs

@@ -1,5 +1,5 @@
 // variables3.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute the command `rustlings hint variables3` if you want a hint :)
 
 fn main() {
     let x = 3;
@@ -7,37 +7,3 @@ fn main() {
     x = 5;
     println!("Number {}", x);
 }
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-// In Rust, variable bindings are immutable by default. But here we're trying
-// to reassign a different value to x! There's a keyword we can use to make
-// a variable binding mutable instead.