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/benches/benchmark.rs

@@ -1,4 +1,4 @@
-use std::fmt::Write as _;
+use core::fmt::Write as _;
 
 use criterion::{BatchSize, Criterion, criterion_group, criterion_main};