chore: fix a bunch of annoying clippy lints

[hawkw]

Motivation

Recent Clippy releases have added some new lints that trigger on some
code in Tokio. These aren't a big deal, but seeing them in my editor is
mildly annoying.

Solution

This branch fixes the following issues flagged by Clippy:

• manual Option::map implementation
• use of .map(...).flatten(...) that could be replaced with
  .and_then(...)
• manual implementation of saturating arithmetic on Durations
• simplify some boolean expressions in assertions (!res.is_ok() can be
  res.is_err())
• fix redundant field names in initializers
• allow the clippy::unnecessary_to_owned lint in ToSocketAddrs impl
  for &[SocketAddr] --- the "unnecessary" to_vec() call is actually
  necessary to ensure the iterator is 'static
• replace an unnecessary cast to usize with an explicitly typed
  integer literal

[Noah-Kennedy]

Looks like clippy is failing.

[hawkw]

Looks like clippy is failing.

Hmm, it looks like it's failing because the clippy::unnecessary_to_owned lint doesn't exist. This seems to be because the Clippy CI workflow is running on Rust 1.52.0 for some reason and that lint wasn't added to Clippy until Rust 1.58.

I'm not sure why the Clippy CI job isn't running on the latest stable Rust, but we seem to have explicitly pinned it to 1.52.0 on purpose...

I guess the simplest solution is to just remove the allow attribute for that lint, so that the clippy CI build passes; users with newer Rust toolchains can just accept that they'll get warnings there, I guess.

[hawkw]

Oh, it looks like the reason Clippy is pinned to 1.52 is to avoid breaking LTS releases when new lints are added: 2cee1db

I'll just go ahead and remove the allow attributes.