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/ime/input_method.rs

@@ -8,7 +8,7 @@ use std::{
 };
 
 use super::{super::atoms::*, ffi, util, XConnection, XError};
-use once_cell::sync::Lazy;
+use crate::utils::Lazy;
 use x11rb::protocol::xproto;
 
 static GLOBAL_LOCK: Lazy<Mutex<()>> = Lazy::new(Default::default);