Flatten children array as it is diffed

[andrewiggins]

Instead of passing over the newVNode's children array twice (once in toChildArray to flatten it, and once again in diffChildren to diff the children), we now pass over the children once, diffing the children and flattening the list as we go.

Total change:
+6 B (22e0fba)
-4 B (ffc1713)

[andrewiggins]

If anyone has the time and setup, would love to know if this PR moves the perf at all

[coveralls]

Coverage increased (+0.0003%) to 99.882% when pulling b7ca4fa on forEachChild into 1d2882e on master.

[JoviDeCroock]

This looks good, in my mind this feels slower because of the function allocation but I think that’s because I think about the classic engines. Should be tested, I’ll see if I can find the big jsperf benchmark on my laptop.

[marvinhagemeister]

So good 👍 💯 Skipping a whole iteration should be faster in theory but I haven't made any tests to get some numbers. @developit maybe you got some benches ready when you suggested the change on slack? 🙂

[cristianbote]

Hey @andrewiggins took a stab at it and tried to resolve the conflicts 🕺 If something it's not right, please let me know.

[developit]

IIRC the first two args here are used externally. Maybe worth changing to toChildArray(children, flattened, callback)?

[marvinhagemeister]

I think that change was made for golfing purposes in children.js. There it's used without the flattened argument. Either way we've only advertised the first arg so far in our types and elsewhere. So I think it's fine to swap them like it's done here.