Remove once_cell

[c410-f3r]

In favor of LazyLock.

Needs Rustc 1.80 that will be released in ten days.

[fitzgen]

Can we refactor things a bit to use std::sync::Once instead, to avoid an aggressive MSRV bump?

[c410-f3r]

Well, at least I tried like in rust-fuzz/arbitrary#112 🤷

Replaced with OnceLock that will be initialized as soon as rust_libfuzzer_debug_path is read in fuzz_target. This behavior differs from the current approach that will initialize only when initialize is called.

[fitzgen]

Thanks!