feat(conversions): add hint comments

exercises/conversions/from_into.rs

@@ -1,6 +1,8 @@
 // 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 {
     name: String,

exercises/conversions/from_str.rs

@@ -4,6 +4,8 @@
 // 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;
 
@@ -37,6 +39,9 @@ enum ParsePersonError {
 //    with something like `"4".parse::<usize>()`
 // 6. If while extracting the name and the age something goes wrong, an error should be returned
 // If everything goes well, then return a Result of a Person object
+//
+// As an aside: `Box<dyn Error>` implements `From<&'_ str>`. This means that if you want to return a
+// string error message, you can do so via just using return `Err("my error message".into())`.
 
 impl FromStr for Person {
     type Err = ParsePersonError;

exercises/conversions/try_from_into.rs

@@ -3,6 +3,8 @@
 // Basically, this is the same as From. The main difference is that this should return a Result type
 // instead of the target type itself.
 // You can read more about it at https://doc.rust-lang.org/std/convert/trait.TryFrom.html
+// Execute `rustlings hint from_str` or use the `hint` watch subcommand for a hint.
+
 use std::convert::{TryFrom, TryInto};
 
 #[derive(Debug, PartialEq)]

exercises/conversions/using_as.rs

@@ -4,6 +4,7 @@
 //
 // The goal is to make sure that the division does not fail to compile
 // and returns the proper type.
+// Execute `rustlings hint using_as` or use the `hint` watch subcommand for a hint.
 
 // I AM NOT DONE