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/option1.rs

@@ -2,7 +2,7 @@
 // This example panics because the second time it calls `pop`, the `vec`
 // is empty, so `pop` returns `None`, and `unwrap` panics if it's called
 // on `None`. Handle this in a more graceful way than calling `unwrap`!
-// Scroll down for hints :)
+// Execute `rustlings hint option1` for hints :)
 
 pub fn pop_too_much() -> bool {
     let mut list = vec![3];
@@ -27,31 +27,3 @@ mod tests {
         assert!(pop_too_much());
     }
 }
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-// Try using a `match` statement where the arms are `Some(thing)` and `None`.
-// Or set a default value to print out if you get `None` by using the
-// function `unwrap_or`.
-// Or use an `if let` statement on the result of `pop()` to both destructure
-// a `Some` value and only print out something if we have a value!