Cleaner implementation

[gabeklein]

Wanted to contribute a refactor I authored while reverse-engineering this module. I'm tinkering with react-virtual and found this to be it's only major dependency. Wanted to in-house logic on my end so iterating off this fork; at some point found a quick PR to be in order.

I've cleaned up the code to be more ES6'y and made it more readable. I've also pared down some unneeded steps and assertions. The algorithm should be a more efficient as a result, which is useful since it will run 60-120ips or more (especially on mobile).

To help with code-review:

• Split discrete improvements made into separate commits
• Changes are non-breaking and API exactly the same.
• Tested using included serve script and works fine.

Feedback welcome!
Cheers