Specify key function

[curran]

The data join key function would be useful to be able to specify, for the sake of efficiency.

[curran]

For example, what if we have two relatively complex components, and a top-level app that renders two of them, first in the order ["a", "b"], then in the order ["b", "a"]. We want to avoid using the default index-based key for the data join here, so we can avoid tearing down and reconstructing both "a" and "b".

Need to come up with a full example, but this feels like an important piece of the puzzle.