Fix various lints

Because this is making our CI fail on main, I am going to set this to merge right away, rather than waiting for absent colleagues to review it. This is without prejudice to reverting any of it in favour of different answers.

In particular see #1176 and #1177.

I have chosen to include the further fixes for nightly and rustdoc, necessary to make CI pass without warnings.

@nickm, please would you do a post-hoc review of this. The relevant commits are 4577399b9..617412713.

Picking on you because, in particular, you may want to do something different to what I did in 91ef1a7d ("clippy: An an allow for fallible ed25519 conversion") (which seems like it might be related to !1767 (merged)).