Clean up and unify exercises

exercises/23_conversions/from_str.rs

@@ -1,13 +1,8 @@
-// from_str.rs
-//
 // This is similar to from_into.rs, but this time we'll implement `FromStr` and
 // return errors instead of falling back to a default value. Additionally, upon
 // implementing FromStr, you can use the `parse` method on strings to generate
 // an object of the implementor type. You can read more about it at
 // https://doc.rust-lang.org/std/str/trait.FromStr.html
-//
-// Execute `rustlings hint from_str` or use the `hint` watch subcommand for a
-// hint.
 
 use std::num::ParseIntError;
 use std::str::FromStr;