Rust-clippy: Potentially redundant/upliftable lints

[clarfonthey]

A few lints I noticed which may be redundant to newer lints in rustc upstream:

• absurd_extreme_comparisons should be uplifted to the existing unused_comparisons
• bad_bit_mask should probably be uplifted to the existing unused_comparisons lint, or an unused_bitmask lint
• bool_comparison should probably be uplifted to unused_comparisons
• cmp_nan should probably be again uplifted to unused_comparisons
• eq_op should probably be again uplifted to unused_comparisons
• unused_collect should be replaced by the existing must_use on collect
• useless_attribute should be replaced by unused_attribute
• out_of_bounds_indexing should be a const eval error
• iterator_step_by_zero should be removed; this now panics
• enum_clike_unportable_variant should be part of a compiler portability lint

(will make larger list when I get around to it)

[Manishearth]

FWIW collect _already_ has must_use, this must have happened in the recentish flurry of PRs adding must_use everywhere. So we can remove this lint if nobody objects.

As for absurd_extreme_comparisons, the clippy lint is better, we should probably uplift the goodies we have.

[clarfonthey]

So as I'm going through a bunch of lints… there are a lot of lints which could just be added to the existing, upstream unused_comparisons lint. A lot of these are super easy to check, with a huge net gain for checking. Plus, I can't see why someone would want to disable them individually, considering how they're very straightforward.

Perhaps the bad_bit_mask lint would be a bit too much for upstream rustc, but a lot of the others would be very good to add upstream.