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/tests/tests3.rs

@@ -1,7 +1,8 @@
 // tests3.rs
 // This test isn't testing our function -- make it do that in such a way that
 // the test passes. Then write a second test that tests whether we get the result
-// we expect to get when we call `is_even(5)`. Scroll down for hints!
+// we expect to get when we call `is_even(5)`.
+// Execute `rustlings hint tests3` for hints :)
 
 pub fn is_even(num: i32) -> bool {
     num % 2 == 0
@@ -16,28 +17,3 @@ mod tests {
         assert!();
     }
 }
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-// You can call a function right where you're passing arguments to `assert!` -- so you could do
-// something like `assert!(having_fun())`. If you want to check that you indeed get false, you
-// can negate the result of what you're doing using `!`, like `assert!(!having_fun())`.