Implement core::iter::Step for PhysFrame and Page

[phil-opp]

This will make it possible to use Rust's native range types instead of our custom PhysFrameRange, PageRange, etc types. Note that we still need to keep these old types as the Step trait is not stable yet.

[phil-opp]

I started the implementation in the step_trait branch. If someone wants to implement this, feel free to continue on top of that branch.

[mental32]

I opened a somewhat related PR #224 that let's *Range types be constructed from Range<u64>.

I'll check out the step_trait branch and see if I can figure out the trait (TIL core::iter::Step exists!)