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/utils.rs

@@ -0,0 +1,30 @@
+// A poly-fill for `lazy_cell`
+// Replace with std::sync::LazyLock when https://github.com/rust-lang/rust/issues/109736 is stablized.
+
+// This isn't used on every platform, which can come up as dead code warnings.
+#![allow(dead_code)]
+
+use std::ops::Deref;
+use std::sync::OnceLock;
+
+pub(crate) struct Lazy<T> {
+    cell: OnceLock<T>,
+    init: fn() -> T,
+}
+
+impl<T> Lazy<T> {
+    pub const fn new(f: fn() -> T) -> Self {
+        Self {
+            cell: OnceLock::new(),
+            init: f,
+        }
+    }
+}
+
+impl<T> Deref for Lazy<T> {
+    type Target = T;
+    #[inline]
+    fn deref(&self) -> &'_ T {
+        self.cell.get_or_init(self.init)
+    }
+}