lazy_static has better ergonomics at the call/access sites (it returns a
reference to the type directly, whereas with once_cell we get a static Lazy<T>
that we must dereference instead) but the once_cell api is slated for
integration into the standard library [0] and has been the "preferred" way to
declare static global variables w/ deferred initialization. It's also less
opaque and easier to comprehend how it works, I guess?
(Both `once_cell` and `lazy_static` are already in our dependency tree, so this
should have no detrimental effect on build times. It actually negligibly
*improves* build times by not using macros, reducing the amount of expansion the
compiler has to do by a miniscule amount.)
[0]: https://github.com/rust-lang/rust/issues/74465
