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_semantics4.rs

@@ -1,7 +1,8 @@
 // move_semantics4.rs
 // Refactor this code so that instead of having `vec0` and creating the vector
 // in `fn main`, we instead create it within `fn fill_vec` and transfer the
-// freshly created vector from fill_vec to its caller. Scroll for hints!
+// freshly created vector from fill_vec to its caller.
+// Execute `rustlings hint move_semantics4` for hints!
 
 fn main() {
     let vec0 = Vec::new();
@@ -25,24 +26,3 @@ fn fill_vec() -> Vec<i32> {
 
     vec
 }
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-// Stop reading whenever you feel like you have enough direction :) Or try
-// doing one step and then fixing the compiler errors that result!
-// So the end goal is to:
-// - get rid of the first line in main that creates the new vector
-// - so then `vec0` doesn't exist, so we can't pass it to `fill_vec`
-// - we don't want to pass anything to `fill_vec`, so its signature should
-//   reflect that it does not take any arguments
-// - since we're not creating a new vec in `main` anymore, we need to create
-//   a new vec in `fill_vec`, similarly to the way we did in `main`