Rust stabilizes zeroed allocation and iterator fixes

Summary

Zeroed allocations are now first-class without unsafe boilerplate; iterator specializations reduce unnecessary work for known-length iterators; fixing infinite loop in `Repeat::last` prevents silent hangs in production code.

Why it matters

Zeroed allocations are now first-class without unsafe boilerplate; iterator specializations reduce unnecessary work for known-length iterators; fixing infinite loop in `Repeat::last` prevents silent hangs in production code.

Sources

1. 1.Box::new_zeroed, Box::new_zeroed_slice, Rc::new_zeroed, Rc::new_zeroed_slice, Arc::new_zeroed, Arc::new_zeroed_slice
2. 2.Specialize Iterator::eq{_by} for TrustedLen iterators
3. 3.iter::Repeat::last and count will now panic, rather than looping infinitely