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

tests/egui_tests/tests/regression_tests.rs

@@ -316,7 +316,7 @@ fn warn_if_rect_changes_id() {
 #[test]
 #[cfg(debug_assertions)]
 fn warn_if_rect_changes_id_false_positive_parent_shift() {
-    use std::cell::Cell;
+    use core::cell::Cell;
 
     let counter = Cell::new(0);
     let button_rect = egui::Rect::from_min_size(egui::pos2(10.0, 10.0), egui::vec2(100.0, 30.0));

tests/egui_tests/tests/test_atoms.rs

@@ -220,8 +220,8 @@ fn test_atom_selectable_senses_click_and_drag() {
 /// See <https://github.com/emilk/egui/issues/8217>.
 #[test]
 fn test_atom_selectable_text_can_be_copied() {
+    use core::cell::Cell;
     use egui::{AtomLayout, Event, Modifiers, OutputCommand, PointerButton, Pos2, Rect};
-    use std::cell::Cell;
 
     fn copied_text(selectable: bool) -> Option<String> {
         let rect_cell = Cell::new(Rect::NOTHING);

tests/egui_tests/tests/test_panel_drag.rs

@@ -381,7 +381,7 @@ fn switched_bottom_panel_harness(start_expanded: bool) -> Harness<'static, Switc
 }
 
 /// Assert that the panel edge crossed `gap` gradually, rather than in one frame.
-fn assert_crossed_gradually(tops: &[f32], gap: std::ops::Range<f32>) {
+fn assert_crossed_gradually(tops: &[f32], gap: core::ops::Range<f32>) {
     let frames_in_gap = tops.iter().filter(|top| gap.contains(top)).count();
     assert!(
         3 <= frames_in_gap,

tests/test_background_logic/src/main.rs

@@ -2,7 +2,7 @@
 #![expect(rustdoc::missing_crate_level_docs)]
 #![allow(clippy::print_stderr)]
 
-use std::time::Duration;
+use core::time::Duration;
 
 use eframe::egui::{self, ViewportInfo};
 
@@ -56,7 +56,7 @@ fn viewport_info(ctx: &egui::Context) -> String {
         ];
         for (name, value) in flags {
             if let Some(value) = value {
-                use std::fmt::Write as _;
+                use core::fmt::Write as _;
                 write!(s, " {name}={value}").ok();
             }
         }

tests/test_inline_glow_paint/src/main.rs

@@ -11,7 +11,7 @@
 use eframe::egui;
 use eframe::glow;
 
-fn main() -> Result<(), Box<dyn std::error::Error>> {
+fn main() -> Result<(), Box<dyn core::error::Error>> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
         renderer: eframe::Renderer::Glow,