update Step::steps_between to latest rust nightly version

[Wasabi375]

This implements the signature change in rust-lang/rust#130867

I also used this opportunity to change the backing implementation to use u64 instead of usize. That way functions like steps_between is always working for Page even on 32bit architectures. The old implementation would return None even if the step count is knowable for Pages but not for VirtAddr.

I wasn't sure whether I should create this PR against master or next. While this is a breaking change it is a breaking change in a nightly feature, where breaking changes are expected. If you want I can easily rebase this on next.