More nightly clippy fixes (2021-09-29)

[sharnoff]

Most of the changes are for the new if-then-panic lint added in rust-lang/rust-clippy#7669. The error messages look like:

error: only a `panic!` in `if`-then statement
   --> zenith_utils/src/lsn.rs:195:9
    |
195 | /         if prev.checked_add(val).is_none() {
196 | |             panic!("AtomicLsn overflow");
197 | |         }
    | |_________^ help: try: `assert!(!prev.checked_add(val).is_none(), "AtomicLsn overflow");`
    |
    = note: `-D clippy::if-then-panic` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#if_then_panic

At first I thought it generally made things less readable, but it actually surprised me - being able to see the assert! at the start of the line makes it abundantly clear what the purpose is.

[SomeoneToIgnore]

The !expected.is_empty() check in conjunction with expected None is quite confusing, but I guess I might not know something 🙂

[sharnoff]

It took me a moment to figure out as well. From a higher level, what the function is doing is really just assert!(result.is_none() == expected.is_empty()) -- it should have returned None iff we weren't expecting anything. This is just the case if we already know the result is Some(_).

https://github.com/zenithdb/zenith/blob/d92412554fe81f22c5d3f52173de3a2e82d9f1d1/pageserver/src/layered_repository/interval_tree.rs#L292-L295