replace the once_cell dependency with the types from libstd

[nagisa]

libstd recently stabilized a number of types that mirror what once_cell crate has been doing. We could replace this crate now with the libstd implementation, thus reducing our direct dependencies by a little bit.

That said this is likely very low impact -- our dependency tree will have crates that in turn depend on once_cell for sure, so the dependency will likely remain in place.