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/tests1.rs

@@ -4,7 +4,7 @@
 // rustlings run --test exercises/tests/tests1.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 tests1` for hints :)
 
 #[cfg(test)]
 mod tests {
@@ -13,37 +13,3 @@ mod tests {
         assert!();
     }
 }
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-// You don't even need to write any code to test -- you can just test values and run that, even
-// though you wouldn't do that in real life :) `assert!` is a macro that needs an argument.
-// Depending on the value of the argument, `assert!` will do nothing (in which case the test will
-// pass) or `assert!` will panic (in which case the test will fail). So try giving different values
-// to `assert!` and see which ones compile, which ones pass, and which ones fail :)