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_semantics3.rs

@@ -1,7 +1,7 @@
 // move_semantics3.rs
 // Make me compile without adding new lines-- just changing existing lines!
 // (no lines with multiple semicolons necessary!)
-// Scroll down for hints :)
+// Execute `rustlings hint move_semantics3` for hints :)
 
 fn main() {
     let vec0 = Vec::new();
@@ -22,24 +22,3 @@ fn fill_vec(vec: Vec<i32>) -> Vec<i32> {
 
     vec
 }
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-// The difference between this one and the previous ones is that the first line
-// of `fn fill_vec` that had `let mut vec = vec;` is no longer there. You can,
-// instead of adding that line back, add `mut` in one place that will change
-// an existing binding to be a mutable binding instead of an immutable one :)