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/tests/tests2.rs

@@ -1,6 +1,6 @@
 // tests2.rs
 // This test has a problem with it -- make the test compile! Make the test
-// pass! Make the test fail! Scroll down for hints :)
+// pass! Make the test fail! Execute `rustlings hint tests2` for hints :)
 
 #[cfg(test)]
 mod tests {
@@ -9,36 +9,3 @@ mod tests {
         assert_eq!();
     }
 }
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-// Like the previous exercise, you don't need to write any code to get this test to compile and
-// run. `assert_eq!` is a macro that takes two arguments and compares them. Try giving it two
-// values that are equal! Try giving it two arguments that are different! Try giving it two values
-// that are of different types! Try switching which argument comes first and which comes second!