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/mod.rs

@@ -11,7 +11,7 @@ use std::{collections::VecDeque, env, fmt};
 use std::{ffi::CStr, mem::MaybeUninit, os::raw::*, sync::Mutex};
 
 #[cfg(x11_platform)]
-use once_cell::sync::Lazy;
+use crate::utils::Lazy;
 use smol_str::SmolStr;
 
 #[cfg(x11_platform)]