fix various clippy nits

[ncloudioj]

Had a chance to try out rust-clippy, it reported a few nits and code simplifications that seemed legit.

Didn't take its suggestion of explicit lifetimes given in parameter types where they could be elided though, I figure perhaps those life time tags were annotated there intentionally.

[mykmelez]

Whoops, sorry for the delay reviewing! It looks like my "watching" status for this repo got reset to "not watching" somehow, so I never received a notice about the PR.

This all looks good, so I'll merge it. Is it possible to include a clippy check in testing, so we avoid re-introducing these nits in the future?

Didn't take its suggestion of explicit lifetimes given in parameter types where they could be elided though, I figure perhaps those life time tags were annotated there intentionally.

They may well have been, although Rust may also have become better at inferring lifetimes, so it's perhaps worth looking at these (in a separate branch).