[Draft] Adding Safe indexing to Array

[gwendal.roue (Gwendal Roué)]

@jarod

I don’t mind either one in isolation – array[index] is a widely-established convention, and it’s intuitive to me that array.at(index) would do the same thing as array[index]

Intuitions can be shaped. array.at(index) has enough potential to enter the basic fundamental Swift toolkit that everybody learns quickly. The official documentation will of course talk about it, since it is so fundamental. All online Swift newbie tutorials will talk about it as well. It will get voted as the best answer about all "index out of bounds" errors on StackOverflow. Etc.