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/functions/functions1.rs

@@ -1,44 +1,6 @@
 // functions1.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute `rustlings hint function1` for hints :)
 
 fn main() {
     call_me();
 }
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-// This main function is calling a function that it expects to exist, but the
-// function doesn't exist. It expects this function to have the name `call_me`.
-// It expects this function to not take any arguments and not return a value.
-// Sounds a lot like `main`, doesn't it?

exercises/functions/functions2.rs

@@ -1,5 +1,5 @@
 // functions2.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute `rustlings hint functions2` for hints :)
 
 fn main() {
     call_me(3);
@@ -10,33 +10,3 @@ fn call_me(num) {
         println!("Ring! Call number {}", i + 1);
     }
 }
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-// Rust requires that all parts of a function's signature have type annotations,
-// but `call_me` is missing the type annotation of `num`.

exercises/functions/functions3.rs

@@ -1,5 +1,5 @@
 // functions3.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute `rustlings hint functions3` for hints :)
 
 fn main() {
     call_me();
@@ -10,33 +10,3 @@ fn call_me(num: i32) {
         println!("Ring! Call number {}", i + 1);
     }
 }
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-// This time, the function *declaration* is okay, but there's something wrong
-// with the place where we're calling the function.

exercises/functions/functions4.rs

@@ -1,5 +1,5 @@
 // functions4.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute `rustlings hint functions4` for hints :)
 
 // This store is having a sale where if the price is an even number, you get
 // 10 (money unit) off, but if it's an odd number, it's 3 (money unit) less.
@@ -20,25 +20,3 @@ fn sale_price(price: i32) -> {
 fn is_even(num: i32) -> bool {
     num % 2 == 0
 }
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-// The error message points to line 12 and says it expects a type after the
-// `->`. This is where the function's return type should be-- take a look at
-// the `is_even` function for an example!

exercises/functions/functions5.rs

@@ -1,5 +1,5 @@
 // functions5.rs
-// Make me compile! Scroll down for hints :)
+// Make me compile! Execute `rustlings hint functions5` for hints :)
 
 fn main() {
     let answer = square(3);
@@ -9,39 +9,3 @@ fn main() {
 fn square(num: i32) -> i32 {
     num * num;
 }
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-// This is a really common error that can be fixed by removing one character.
-// It happens because Rust distinguishes between expressions and statements: expressions return
-// a value based on its operand, and statements simply return a () type which behaves just like `void` in C/C++ language.
-// We want to return a value of `i32` type from the `square` function, but it is returning a `()` type...
-// They are not the same. There are two solutions:
-// 1. Add a `return` ahead of `num * num;`
-// 2. remove `;`, make it to be `num * num`