Clean up and unify exercises

exercises/23_conversions/from_into.rs

@@ -1,11 +1,6 @@
-// from_into.rs
-//
 // The From trait is used for value-to-value conversions. If From is implemented
 // correctly for a type, the Into trait should work conversely. You can read
 // more about it at https://doc.rust-lang.org/std/convert/trait.From.html
-//
-// Execute `rustlings hint from_into` or use the `hint` watch subcommand for a
-// hint.
 
 #[derive(Debug)]
 struct Person {
@@ -24,7 +19,6 @@ impl Default for Person {
     }
 }
 
-
 // Your task is to complete this implementation in order for the line `let p1 =
 // Person::from("Mark,20")` to compile. Please note that you'll need to parse the
 // age component into a `usize` with something like `"4".parse::<usize>()`. The