Multi height multi view

- [fixes #87, #56]

- [x] support multiple different view types
- [x] first pass support for multiple heights
- [x] _numChildViewsForViewport ("seeem to work")
- [x] _startingIndex ("seeeeeem to work")
- [x] memoization for height
- [x] much cleanup and refactoring
- [x] example
- [ ] website
- [x] in some scenarios we have a math problem, that results in a flicker... Lets fix it.

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Bonus

- [ ] pooling of different view types
- [ ] grid should work, but each row should be the height of its largest entry
- [ ] support dynamic heights (where they can change at anytime)