fix(clippy): nested ifs/let chains

[jlizen]

What does this PR do?

Initial pass to address clippy lints, before adding to to CI.

This one has a few because they would have caused merge conflicts otherwise:

• Collapse nested ifs into let chains
• manual_flatten
• dead_code
• option_as_ref_deref

All seem reasonable to me

Disclosure questions

AI disclosure.

• I used an AI tool for research, autocomplete, or in other minimal ways
• The AI tool authored small parts of the code (e.g., autocomplete, comments)
• The AI tool authored large parts of the code

Confidence level.

• I am very happy with it

[jlizen]

@jackh726 this does indeed look like dead code, but is it supposed to be?

[jackh726]

shrug idk, I think Niko (or Claude) wrote this code.

[nikomatsakis]

All these changes look good to me. Exciting that if foo && let is stable now, I keep forgetting. But before I merge, I'd like to hear from @jackh726 on that dead code question.