Replace lazy_static with OnceLock inside monero-serai

lazy_static, if no_std environments were used, effectively required always
using spin locks. This resolves the ergonomics of that while adopting Rust std
code.

no_std does still use a spin based solution. Theoretically, we could use
atomics, yet writing our own Mutex wasn't a priority.

common/std-shims/src/lib.rs

@@ -6,6 +6,7 @@
 #[macro_use]
 pub extern crate alloc;
 
+pub mod sync;
 pub mod collections;
 pub mod io;