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/macros2.rs

@@ -1,5 +1,5 @@
 // macros2.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute `rustlings hint macros2` for hints :)
 
 fn main() {
     my_macro!();
@@ -10,64 +10,3 @@ macro_rules! my_macro {
         println!("Check out my macro!");
     };
 }
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-// Macros don't quite play by the same rules as the rest of Rust, in terms of
-// what's available where.
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-// Unlike other things in Rust, the order of "where you define a macro" versus
-// "where you use it" actually matters.