v8: reimplement getHeapStatistics

[vkurchatkin]

Reimplement getHeapStatistics without smalloc as it is going to be removed soon.

R=@trevnorris @bnoordhuis

[bnoordhuis]

This seems much more convoluted than it needs to be. Why don't you just new Uint32Array(v8binding.kHeapStatisticsBufferLength) in lib/v8.js?

[trevnorris]

How @bnoordhuis suggested is exactly how I was planning on reimplementing it with the new Buffer implementation. But, thanks for taking it off my plate. :)

[vkurchatkin]

No problem! I still have to store buffer pointer on env to clean up properly

[bnoordhuis]

I'm not sure I understand why. If you're worried about externalizing the array, you can use typed_array->Set(index, ...). It's not super fast but you'll only be setting a handful of values.

On second thought, you could even use a regular array. I doubt anyone will notice the difference.

[vkurchatkin]

This should be covered by #2022