Fix or disable recent nightly clippy lints

[teor2345]

Motivation

Some developers use nightly, and we want builds to run without clippy warnings.

Some clippy lints provide useful refactors.

Solution

• Turn if ... panic!(...) into assert!
• Remove redundant closures
• Disable mutable collection key lint

It is a logic error for an item to be modified in such a way that the item’s ordering relative to any other item, as determined by the Ord trait, changes while it is in the set.

Review

Anyone can review this PR. It's not urgent.

Reviewer Checklist

• Mutable collection key usage is correct
  • https://rust-lang.github.io/rust-clippy/master/index.html#mutable_key_type
  • https://doc.rust-lang.org/std/collections/struct.BTreeSet.html
• Existing tests pass

[conradoplg]

Looks good

[dconnolly]

👍