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/macros/macros1.rs

@@ -1,5 +1,5 @@
 // macros1.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute `rustlings hint macros1` for hints :)
 
 macro_rules! my_macro {
     () => {
@@ -10,55 +10,3 @@ macro_rules! my_macro {
 fn main() {
     my_macro();
 }
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-// When you call a macro, you need to add something special compared to a
-// regular function call. If you're stuck, take a look at what's inside
-// `my_macro`.