if2 exercise and option1 fix

[wrobstory]

This PR implements two things:

1. A new if2 exercise that shows the type of compiler message you get if your conditional branches don't return the same type as well as the usage of else if
2. A small change to the option1 exercise that I think makes it very confusing (no problem dropping this one if there's disagreement here).

[wrobstory]

Happy to discuss this; here's the compiler message you get:

21 |         numbers[iter] = number_to_add;
   |         ^^^^^^^^^^^^^ slice indices are of type `usize` or ranges of `usize`
   |
   = help: the trait `std::slice::SliceIndex<[std::option::Option<u16>]>` is not implemented for `u16`
   = note: required because of the requirements on the impl of `std::ops::Index<u16>` for `[std::option::Option<u16>]`

The message makes it sound like it could have something to do with a SliceIndex of Option, but really this is about the type of the slice index. While this is certainly a learning opportunity, it's not one that has anything to do with Option usage.

I don't feel super strongly about it, but it was the place I got stuck the longest in the exercises!

[sanjaykdragon]

as the creator of the option1 exercise, you are correct and this change looks good. Looks like I forgot to add the "as usize" myself

[wrobstory]

Thanks all!

[shadows-withal]

@all-contributors add @wrobstory for code

[allcontributors]

@fmoko

I've put up a pull request to add @wrobstory! 🎉