Day 2: Idiomatic num_divisors() implementation

[xmartinez]

Provide an alternative implementation for num_divisors that:

• uses map and product from the Iterator trait (instead of fold),
• uses map and ok from the Result trait (instead of pattern matching on Ok and Err).

Also, update the included line ranges in day2.md accordingly (some of them were outdated).

[xmartinez]

I am not 100% sure that the proposed change is actually very idiomatic (I am still learning Rust), but I find the introduction of the new methods (map, product, ok) useful for learning the Result and Iterator traits.

BTW, thanks a lot for making this docs! I am finding them very helpful while getting familiar with Rust.

[xmartinez]

The Travis CI failure is unrelated to this pull request. It is timing out in the day20 program when fetching an external URL (http://pokeapi.co/api/v2/pokemon/111).

[xmartinez]

Closed/reopened PR to trigger CI builds again.

[zsiciarz]

Awesome, thanks! I wrote this part before 1.0, when product() was in an unstable MultiplicativeIterator trait and hence I used fold() here. But you're absolutely right about product() being idiomatic Rust today.