v8: optimize `getHeapStatistics`

Since setting object properties in C++ can be slow, pass
data to JS using preallocated smalloc buffer and create
object in JS instead.

PR-URL: https://github.com/iojs/io.js/pull/469


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>