Assorted changes to fix warnings & remove dead code

[grjte]

This PR does some minor tidying up:

• The div_ceil refactor (commit 8f819a0) removes the necessity of declaring the unstable int_roundings feature in order for this repo to used externally. See Tracking Issue for int_roundings rust-lang/rust#88581
• all other commits remove dead code and unused or broken tests, or resolve clippy warnings. In the case of imports, I updated them to follow a consistent pattern throughout the repo.

The commits are very granular to make it very easy to review/update if there's anything you want to do differently (e.g. maybe you prefer to ignore needless_range_loop clippy warnings). Happy to squash before merge if you prefer

[therealyingtong]

LGTM!