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/errors1.rs

@@ -4,7 +4,7 @@
 // was, instead of just sometimes returning `None`. The 2nd test currently
 // does not compile or pass, but it illustrates the behavior we would like
 // this function to have.
-// Scroll down for hints!!!
+// Execute `rustlings hint errors1` for hints!
 
 pub fn generate_nametag_text(name: String) -> Option<String> {
     if name.len() > 0 {
@@ -38,36 +38,3 @@ mod tests {
         );
     }
 }
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-// `Err` is one of the variants of `Result`, so what the 2nd test is saying
-// is that `generate_nametag_text` should return a `Result` instead of an
-// `Option`.
-
-// To make this change, you'll need to:
-// - update the return type in the function signature to be a Result<String, String> that
-//   could be the variants `Ok(String)` and `Err(String)`
-// - change the body of the function to return `Ok(stuff)` where it currently
-//   returns `Some(stuff)`
-// - change the body of the function to return `Err(error message)` where it
-//   currently returns `None`
-// - change the first test to expect `Ok(stuff)` where it currently expects
-//   `Some(stuff)`.