[TODO] Use `ilog` when MSRV is 1.67 or greater

[prestwich]

nit: can we add a TODO here to use ilog when upgrading MSRV to 1.67 or greater?

Originally posted by @prestwich in #488 (comment)

[DaniPopes]

Doesn't really optimize that well https://godbolt.org/z/fezfcjWxP