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/error_handling/result1.rs

@@ -1,5 +1,5 @@
 // result1.rs
-// Make this test pass! Scroll down for hints :)
+// Make this test pass! Execute `rustlings hint option2` for hints :)
 
 #[derive(PartialEq, Debug)]
 struct PositiveNonzeroInteger(u64);
@@ -25,22 +25,3 @@ fn test_creation() {
     );
     assert_eq!(Err(CreationError::Zero), PositiveNonzeroInteger::new(0));
 }
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-// `PositiveNonzeroInteger::new` is always creating a new instance and returning an `Ok` result.
-// It should be doing some checking, returning an `Err` result if those checks fail, and only
-// returning an `Ok` result if those checks determine that everything is... okay :)