Add `product_ok` and `sum_ok`

[sjackman]

• .try_product() is a more convenient way of writing .product::<Result<_, _>>()
• .try_sum() is a more convenient way of writing .sum::<Result<_, _>>()

Modelled after .try_collect()

[Philippe-Cholet]

We have a bunch of methods about iterators of results: map_ok filter_ok filter_map_ok flatten_ok fold_ok (and related but different process_results) and try_collect clashing with the nightly Iterator::try_collect.

I think try_ methods should be reserved to (unstable for now) Try trait usages. For consistency, I would suggest names to be product_ok and sum_ok.

Now, do we want to add those shortcuts? That's the question. @jswrenn What do you think?

EDIT: The documentation of sum (same for product) mention panics. It would do the same here.

Panics

When calling sum() and a primitive integer type is being returned, this method will panic if the computation overflows and debug assertions are enabled.

I failed to read the documentation once but if this is not mentioned then someone will have a problem.

[sjackman]

For consistency, I would suggest names to be product_ok and sum_ok.

Done!

[sjackman]

EDIT: The documentation of sum (same for product) mention panics. It would do the same here.

Done!

[Philippe-Cholet]

@jswrenn Merry Xmas. What do you think about adding product_ok and sum_ok? (There is also #830 about a fold_while_ok?)