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/lib.rs

@@ -187,6 +187,7 @@ mod icon;
 pub mod keyboard;
 pub mod monitor;
 mod platform_impl;
+mod utils;
 pub mod window;
 
 pub mod platform;