Rollup merge of #147751 - nnethercote:bit_set-Word, r=Zalathar

Use `bit_set::Word` in a couple more places.

It's a synonym for `u64` and there are a couple of places where we use `u64` where we should use `Word`, which this commit fixes.

I found this when I tried changing `Word` to `u128` (which made performance worse).

r? `````@Zalathar`````

Author: matthiaskrgr