style: switch instances of `allow` to `expect`

[GregoryConrad]

Fixes #58

[GregoryConrad]

Note to future self:

This will fail for the next couple months (until Rust 1.81 stable is released). At that point, rerun the CI here and it should be green. Or maybe not, which means we need to update the MSRV to 1.81 (in Cargo.toml, README, GitHub ci files).