Some more cleanup in core

[WaffleLapkin]

• remove some integer casts from slice iter (proposed in Make use of [wrapping_]byte_{add,sub} #100819 (comment))
• replace as usize casts with usize::from in slice sort (proposed in Replace most uses of pointer::offset with add and sub #100822 (comment))

r? @scottmcm

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[scottmcm]

Thanks!

@bors r+ rollup

[bors]

📌 Commit 5a67292 has been approved by scottmcm

It is now in the queue for this repository.