Add CheckedSum and CheckedProduct traits

[wainwrightmark]

Closes #250

This adds CheckedSum and CheckedProduct traits as well as blanket implementations for them.

It also adds CheckedSumIter and CheckedProductIter traits and blanket implementations for those as that provides a much more ergonomic interface, allowing you to do:

asserteq!(Some(42), [40,2].iter().checked_sum());
asserteq!(Some(42), [40,2].into_iter().checked_sum());

[cuviper]

Rather than a new iter module here, I think it would make more sense to move this to the num-iter crate.

[cuviper]

This is opposite of how Product is defined, where the type parameter is the input (Item) and it always outputs Self. I would rather match that for consistency, and same for Sum.

[wainwrightmark]

I completely agree. I feel very silly for having done it the other way around. I have changed it now.

[cuviper]

Similarly, for consistency with Iterator, this trait should have no parameters, and the method should be parameterized checked_product<P: CheckedProduct<Self::Item>>.

I think then we could also combine your two iterator extensions into one CheckedIterator trait, or even NumIterator to leave room for more extension methods.

[wainwrightmark]

Another great idea, thankyou. I have done this and put both into NumIter

[wainwrightmark]

Rather than a new iter module here, I think it would make more sense to move this to the num-iter crate.

I wasn't sure. I'm very happy to close this pull request and move the code changes over there if that's best.