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/enums/enums1.rs

@@ -1,5 +1,5 @@
 // enums1.rs
-// Make me compile! Scroll down for hints!
+// Make me compile! Execute `rustlings hint enums1` for hints!
 
 #[derive(Debug)]
 enum Message {
@@ -12,31 +12,3 @@ fn main() {
     println!("{:?}", Message::Move);
     println!("{:?}", Message::ChangeColor);
 }
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-// Hint: The declaration of the enumeration type has not been defined yet.