Use Self everywhere (#3787)

This turns on the clippy lint
[`clippy::use_self`](https://rust-lang.github.io/rust-clippy/v0.0.212/index.html#use_self)
and fixes it everywhere.

crates/egui_demo_app/src/apps/image_viewer.rs

@@ -27,9 +27,9 @@ enum ChosenFit {
 impl ChosenFit {
     fn as_str(&self) -> &'static str {
         match self {
-            ChosenFit::ExactSize => "exact size",
-            ChosenFit::Fraction => "fraction",
-            ChosenFit::OriginalSize => "original size",
+            Self::ExactSize => "exact size",
+            Self::Fraction => "fraction",
+            Self::OriginalSize => "original size",
         }
     }
 }