Batch collection view updates

[JosephDuffy]

Replacing composed-swift/ComposedUI#15

[JosephDuffy]

@shaps80 could use your help on this one. This is a very complex change but it improves performance a lot when performing a lot of updates.

I think it needs more tests, and a couple of them need to be fixed.

This could become a big selling point for Composed but it need to be correct!

[JosephDuffy]

This is my biggest challenge at the moment with Composed. I think there's promise in going down the route of checking which indexes are "available" (like how removeElements(at:) works) but having some proper terms for this and creating a more generalised version is currently stumping me.

Any ideas/tests/fixes would be very welcome @shaps80!

[JosephDuffy]

There are parts that could be improved (reloading sections for example) but the general structure and logic is done now.

More tests are welcome but I don't have any specific scenarios that I know don't work.

[shaps80]

Hey sorry I didn't have time recently to look into this when you asked. I'll review today 100%