Provide alternatives for std::lazy::SyncLazy type?

[phil-opp]

RFC rust-lang/rfcs#2788 introduced a (still unstable) std::lazy module. While the Lazy and OnceCell types of this module are also available in core, their Sync counterparts are not because of their Mutex dependency.

The spin crate already provides a spin::Once as an alternative to the std::lazy::SyncOnceCell type (albeit with a different API). It would be useful if it would also provide an alternative to the std::lazy::SyncLazy type. This way, we could avoid depending on the lazy_static crate.

[zesterer]

Do you think this could be implemented as a thin layer on top of Once, or perhaps even integrated into it? The only obstacle to that I can forsee is the take method but I could easily add support for something like that to Once.

[phil-opp]

Yeah, I think it could be a thin wrapper around Once. I can prepare a PR if you like.

I don't think that the take method is needed for that, but it might be useful on its own.

[phil-opp]

I opened a PR at #89. It's just a copy of the standard library implementation, so please let me know if you think that it needs some adjustments.

[zesterer]

Perhaps adjustments might be required in the future, but for now this good enough for inclusion. Thanks!