Add equivalent of once_cell::sync::Lazy (or std::lazy::SyncLazy)

[taiki-e]

loom currently provides lazy_static macro, but tokio and others are moving to once_cell (e.g., tokio-rs/tokio#3187). It would be nice if loom could provide an equivalent of once_cell::sync::Lazy or std::lazy::SyncLazy.

[taiki-e]

Closing in favor of #117.