I like to run nightly clippy every so often to make our future rust
upgrades easier. Some notable changes:
* Prefer `next_back()` over `last()`. Generic iterators will implement
`last()` to run forward through the iterator until the end.
* Prefer `io::Error::other()`.
* Use implicit returns
One case where I haven't dealt with the issues is the now
[more-sensitive "large enum variant"
lint](https://github.com/rust-lang/rust-clippy/pull/13833). I chose not
to take any decisions around it here, and simply marked them as allow
for now.
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