`CellMeasurer` now properly uses `shallowCompare`.
It also supports a custom caching strategy for measured cell sizes (see `cellSizeCache` prop).

`Collection` supports 2 new properties: `horizontalOverscanSize` and `verticalOverscanSize`.
These properties enable the `Collection` to "overscan" its content similar to how `Grid` does.
This can reduce flicker around the edges when a user scrolls quickly.
