Chrome poor performance when calling array.unshift on a large array.

[lowaa]

I'm really enjoying preact, thanks for developing the project!

I noticed when quickly adding and removing a large amount of nodes, Chrome 68 experiences poor performance. Using the profiler, most of the time is spent in mounts.unshift(). This PR changes mounts.unshift() to mounts.push() and reversing the order that mounts are flushed to maintain functionality.

This project demonstrates the issue: https://github.com/lowaa/preact-perf

With preact 8.3.1, Chrome 68 running on a macbook pro 2018:

After the change:

The scripting time is significantly decreased but maximum heap usage goes up by ~25%

Also tested on Firefox 62 and Edge 17. These browsers did not seem to have an issue with the use of array.unshift(). The change to array.push also seemed to have no effect.

[coveralls]

Coverage remained the same at 100.0% when pulling 9b81167 on lowaa:remove-mounts-unshift into b5c4a36 on developit:master.

[marvinhagemeister]

@lowaa Woa, that's dope 👍 👍 I love the detailed description of this PR and can confirm that this is noticeably faster due to not having to resize the array all the time 💯