* chore: remove `std::mem::*` imports now unnecessary with `REPO_MSRV`
`std::mem::{size,align}_of{,_val}` was added to `std::prelude` in Rust
1.80; see
[`rust`#123168](https://github.com/rust-lang/rust/pull/123168/).
* refactor(benches): s/once_cell::Lazy/std::sync::LazyLock
Weaken our dependence on the `once_cell` crate by using functionality
from `std` instead that was upstreamed from `once_cell`, this time with
what's available in Rust 1.80+.
It's not yet possible to eliminate this dependency entirely, but do what
we can with `REPO_MSRV` for now.
* chore: remove unnecessarily `allow`'d lint rules under `REPO_MSRV`
* chore: migrate easy `allow`s to `expect` under `REPO_MSRV`
Remove or `expect` clear-cut `allow` statements that were easy for me to
figure out.
* chore: `warn` on `clippy::allow_attributes` under `REPO_MSRV`
