Clean up rustc and clippy warnings.

[anforowicz]

This addresses the warnings reported in
https://github.com/mozilla/cargo-vet/actions/runs/8444313897/job/23159306099

For more detailsa about the rationale for the clippy-related changes, see:

• https://rust-lang.github.io/rust-clippy/master/index.html#/suspicious_open_options
• https://rust-lang.github.io/rust-clippy/master/index.html#/thread_local_initializer_can_be_made_const

[anforowicz]

AFAIU tokio::sync::SemaphorePermit is not supposed to be read:

• We don't need to execute it's forget or merge methods
• We do care about the implicitly executed drop method

Not sure if some additional follow-up may be needed here. I wondered about reporting this at https://github.com/rust-lang/rust/issues/new, but I guess not all drop methods count as using a field (Vec's drop probably wouldn't count). So I guess just allowing "dead code" here is a reasonable workaround.

[anforowicz]

Ooops... I didn't run cargo fmt... This should be fixed now in 008636f