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_semantics2.rs

@@ -1,5 +1,6 @@
 // move_semantics2.rs
-// Make me compile without changing line 10! Scroll down for hints :)
+// Make me compile without changing line 10!
+// Execute `rustlings hint move_semantics2` for hints :)
 
 fn main() {
     let vec0 = Vec::new();
@@ -23,31 +24,3 @@ fn fill_vec(vec: Vec<i32>) -> Vec<i32> {
 
     vec
 }
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-// So `vec0` is being *moved* into the function `fill_vec` when we call it on
-// line 7, which means it gets dropped at the end of `fill_vec`, which means we
-// can't use `vec0` again on line 10 (or anywhere else in `main` after the
-// `fill_vec` call for that matter). We could fix this in a few ways, try them
-// all!
-// 1. Make another, separate version of the data that's in `vec0` and pass that
-// to `fill_vec` instead.
-// 2. Make `fill_vec` borrow its argument instead of taking ownership of it,
-// and then copy the data within the function in order to return an owned
-// `Vec<i32>`
-// 3. Make `fill_vec` *mutably* borrow its argument (which will need to be
-// mutable), modify it directly, then not return anything. Then you can get rid
-// of `vec1` entirely -- note that this will change what gets printed by the
-// first `println!`