Fix std::ops::Range size_hint and ExactSizeIterator impls

Fix std::ops::Range size_hint and ExactSizeIterator impls

When self.start > self.end, these iterators simply return None,
so we adjust the size_hint to just return zero in this case.

Certain optimizations can be implemented in and outside libstd if we
know we can trust the size_hint for all inputs to for example
Range<usize>.

This corrects the ExactSizeIterator implementations, which IMO were
unsound and incorrect previously, since they allowed a range like (2..1)
to return a size_hint of -1us in when debug assertions are turned off.