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/macros4.rs

@@ -1,5 +1,5 @@
 // macros4.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute `rustlings hint macros4` for hints :)
 
 macro_rules! my_macro {
     () => {
@@ -14,64 +14,3 @@ fn main() {
     my_macro!();
     my_macro!(7777);
 }
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-// You only need to add a single character to make this compile.
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-// The way macros are written, it wants to see something between each
-// "macro arm", so it can separate them.