Migrate from lazy_static to once_cell

[james7132]

lazy_static involves macros, takes a bit longer to compile, and once_cell provides more flexible ways on the way to being added to std. Replace all usages of lazy_static with static VAR: Lazy<T>.

One potential drawback is that lazy_static supports spin-waiting on the result, while Lazy doesn't. I don't think any of the crate's usages are perf-bound by this though.

[notgull]

The spin crate supports spinning on the result with a spin::OnceCell, which you can use on a feature flag.