m: Remove once_cell dependency

Removes the once_cell dependency, instead using std::sync::OnceLock and a
minimal polyfill for std::sync::LazyLock, which may be stabilized soon
(see rust-lang/rust#121377).

This should not require a bump in MSRV, as OnceLock was stabilized in 1.70,
which this crate is using.

src/platform_impl/linux/x11/util/wm.rs

@@ -1,6 +1,6 @@
 use std::sync::Mutex;
 
-use once_cell::sync::Lazy;
+use crate::utils::Lazy;
 
 use super::*;