Cleanup `NumBuffer` comment and replace `ilog(10)` with `ilog10()` A [nice person from mastodon](https://toot.cat/@jamey/116815991086205506) pointed out that a `FIXME` comment should have been removed alongside the others in https://github.com/rust-lang/rust/pull/157976, and also pointed out that the documentation mentions that `ilog10` is more optimized than `ilog(10)`. Shouldn't matter much here but since I already made a PR to remove the FIXME comment... r? @Amanieu