Uniform lists only need to calculate item size once

[ltgorm]

Observed that the getItemSizeAndItemsPerRow was at the top of cpu hogs when profiling for traversal of a large list. This change reduces the time spent on getting those attributes.

[caseywebdev]

I like the idea, but this is currently a breaking change. At the moment, uniform means all elements are the same size for a given render, but not necessarily the same size for the life of the component. As such, this code would break the case for column sizes that change on window resize or elements that resize responsively. If the performance difference is that significant, it may be worth implementing this behind a new prop (strictlyUniform? staticUniform? or something) that will only ever calculate the item size and items per row once.

[caseywebdev]

Check out https://github.com/orgsync/react-list/tree/use-static-size for a backwards-compatible option