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 :)

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!

exercises/tests/tests3.rs

@@ -1,7 +1,8 @@
 // tests3.rs
 // This test isn't testing our function -- make it do that in such a way that
 // the test passes. Then write a second test that tests whether we get the result
-// we expect to get when we call `is_even(5)`. Scroll down for hints!
+// we expect to get when we call `is_even(5)`.
+// Execute `rustlings hint tests3` for hints :)
 
 pub fn is_even(num: i32) -> bool {
     num % 2 == 0
@@ -16,28 +17,3 @@ mod tests {
         assert!();
     }
 }
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-// You can call a function right where you're passing arguments to `assert!` -- so you could do
-// something like `assert!(having_fun())`. If you want to check that you indeed get false, you
-// can negate the result of what you're doing using `!`, like `assert!(!having_fun())`.