Replace impl Into<String> with impl ToString (#302)

* 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

epaint/src/shape.rs

@@ -124,15 +124,16 @@ impl Shape {
         }
     }
 
+    #[allow(clippy::needless_pass_by_value)]
     pub fn text(
         fonts: &Fonts,
         pos: Pos2,
         anchor: Align2,
-        text: impl Into<String>,
+        text: impl ToString,
         text_style: TextStyle,
         color: Color32,
     ) -> Self {
-        let galley = fonts.layout_multiline(text_style, text.into(), f32::INFINITY);
+        let galley = fonts.layout_multiline(text_style, text.to_string(), f32::INFINITY);
         let rect = anchor.anchor_rect(Rect::from_min_size(pos, galley.size));
         Self::Text {
             pos: rect.min,