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/modules/modules2.rs

@@ -1,5 +1,5 @@
 // modules2.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute `rustlings hint modules2` for hints :)
 
 mod delicious_snacks {
     use self::fruits::PEAR as fruit;
@@ -23,25 +23,3 @@ fn main() {
         delicious_snacks::veggie
     );
 }
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-// The delicious_snacks module is trying to present an external
-// interface (the `fruit` and `veggie` constants) that is different than
-// its internal structure (the `fruits` and `veggies` modules and
-// associated constants). It's almost there except for one keyword missing for
-// each constant.