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/modules1.rs

@@ -1,5 +1,5 @@
 // modules1.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute `rustlings hint modules1` for hints :)
 
 mod sausage_factory {
     fn make_sausage() {
@@ -10,34 +10,3 @@ mod sausage_factory {
 fn main() {
     sausage_factory::make_sausage();
 }
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-// Everything is private in Rust by default-- but there's a keyword we can use
-// to make something public! The compiler error should point to the thing that
-// needs to be public.