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/error_handling/errors2.rs

@@ -14,7 +14,7 @@
 // and add.
 
 // There are at least two ways to implement this that are both correct-- but
-// one is a lot shorter! Scroll down for hints to both ways.
+// one is a lot shorter! Execute `rustlings hint errors2` for hints to both ways.
 
 use std::num::ParseIntError;
 
@@ -43,27 +43,3 @@ mod tests {
         );
     }
 }
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-// One way to handle this is using a `match` statement on
-// `item_quantity.parse::<i32>()` where the cases are `Ok(something)` and
-// `Err(something)`. This pattern is very common in Rust, though, so there's
-// a `?` operator that does pretty much what you would make that match statement
-// do for you! Take a look at this section of the Error Handling chapter:
-// https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
-// and give it a try!