Fix some clippy lint.

[Foreverhighness]

• non_canonical_partial_ord_impl

It seems like partial_cmp never return None, so I made small change of it.
The new implementation has a small logic change from the previous one, since arguments passed to Ordering::then are eagerly evaluated.
I pefer it because it's more succinct than Ordering::then_with and without notable performace penalty. That's enough for an educational project.

• manual_clamp

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My rust version is rustc 1.79.0 (129f3b996 2024-06-10), so there are new clippy warnnings, list below.

• assigning_clones

It looks lik assigning_clones is allowed by default from the webpage, but when I run cargo clippy, I get a warning.

• legacy_numeric_constants
• missing_transmute_annotations

[skyzh]

Thanks a lot!