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/variables1.rs

@@ -1,42 +1,7 @@
 // variables1.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute the command `rustlings hint variables1` if you want a hint :)
 
 fn main() {
     x = 5;
     println!("x has the value {}", x);
 }
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-// Hint: The declaration on line 5 is missing a keyword that is needed in Rust
-// to create a new variable binding.