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/move_semantics/move_semantics1.rs

@@ -1,5 +1,5 @@
 // move_semantics1.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute `rustlings hint move_semantics1` for hints :)
 
 fn main() {
     let vec0 = Vec::new();
@@ -22,21 +22,3 @@ fn fill_vec(vec: Vec<i32>) -> Vec<i32> {
 
     vec
 }
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-// So you've got the "cannot borrow immutable local variable `vec1` as mutable" error on line 11,
-// right? The fix for this is going to be adding one keyword, and the addition is NOT on line 11
-// where the error is.