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/variables2.rs

@@ -1,5 +1,5 @@
 // variables2.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute the command `rustlings hint variables2` if you want a hint :)
 
 fn main() {
     let x;
@@ -9,39 +9,3 @@ fn main() {
         println!("Not ten!");
     }
 }
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-// The compiler message is saying that Rust cannot infer the type that the
-// variable binding `x` has with what is given here.
-// What happens if you annotate line 5 with a type annotation?
-// What if you give x a value?
-// What if you do both?
-// What type should x be, anyway?
-// What if x is the same type as 10? What if it's a different type?