* Replace `impl Into<String>` with `impl ToString`
This is something I ran into today. Types that implement
`std::fmt::Display` cannot be passed to functions that take `impl
Into<String>`. You have to call `display_thing.to_string()`. Its a small
thing but would be fixed by instead taking `impl ToString`.
Afaik `impl ToString` is a superset of `impl Into<String>`, unless users
manually implement `Into<String> for T` (or `From<T> for String`) for
their own types. However I think its more common to implement `Display`
as that works with `println` and friends. The main difference is that
`Display::fmt` can return errors but thats also quite rare in my
experience.
I did some testing in a [playground] and seems to work.
[playground]: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=1111e071f6ae416ae2688d58d2e9b575
* Silence warnings
