Add Step::forward/backward_overflowing to enable RangeInclusive loop optimizations

[pitaj]

ACP: rust-lang/libs-team#767

This adds new required methods to the Step trait:

trait Step: ... {
    // ... existing functions

    // New required functions
    fn forward_overflowing(start: Self, count: usize) -> (Self, bool);
    fn backward_overflowing(start: Self, count: usize) -> (Self, bool);
}

It was found that using these to implement RangeInclusive's Iterator impl enabled optimizations previously only applicable to the exclusive-ended Range.

This required changing how "exhaustion" works for RangeInclusive. I've nominated this for libs-api discussion because of one insta-stable change:

PartialEq impl now always returns false if either RangeInclusive is exhausted
I changed this because the new implementations now only set exhausted when overflow occurs, and start is now advanced past end otherwise. I doubt anyone depends on the prior behavior, but it's probably worth a crater run.

The exhaustion changes also affect Debug but my understanding is that debug formatting is never guaranteed stable.

Note: I did not change the nth impls to use the new functions yet. I want buyoff on these changes first. If the changes to legacy ops::RangeInclusive are rejected, I will change this PR to target the new range::RangeInclusive instead.

r? libs