add, test, and document at

[AlexVipond]

For at, I decided to support both positive and negative indices, since negative indices are such a popular feature of Array.prototype.at.

When the index is positive, it uses find under the hood for optimal performance. When the index is negative, it basically works the same way as reverse—convert the whole iterator to an array, and fall back to Array.prototype.at under the hood.

[RobinMalfait]

Hey, thanks so much for this contribution! 🙏

I rebased the branch and made some adjustments in #30, but made sure to still include you as a contributor. I also made a recent change such that an insiders release is published so that you can already play with it:

• npm install lazy-collections@insiders