Run cargo +nightly fmt to format many let ... else ...

rustfmt has grown opinions about how let ... else ... ought to be formatted. They don't always agree with our previous manual decisions.

I think our policy is to always insist on rustfmt.[1] When that version of rustfmt hits stable, our CI will start to fail for everyone. (Right now this discrepancy just causes trouble for contributors who are using nightly by default.)

In this MR I make no effort to ensure this is kept up to date. I don't think we want to be running a nightly fmt in CI, so we'll live with this slightly anomalous situation until the feature makes it to stable Rust.

[1] For the avoidance of doubt: I don't like rustfmt; if everything were up to me, it wouldn't be widely used in the Rust community, and then we wouldn't use it either. But I don't want to litigate that here.

(edited to add missing "don't", oops)