Enable the clippy::std_instead_of_core lint (#8394)

Prefer `core::` over `std::` where either work

* Part of https://github.com/emilk/egui/issues/5735

crates/egui_demo_lib/src/demo/tests/input_test.rs

@@ -123,7 +123,7 @@ impl crate::View for InputTest {
 }
 
 fn response_summary(response: &egui::Response, show_hovers: bool) -> String {
-    use std::fmt::Write as _;
+    use core::fmt::Write as _;
 
     let mut new_info = String::new();