fs: PR-fix. Fix use of node_isolate.

As suggested by @indutny.
fs: PR-fix. Do not state array size in definition.

As suggested by @indutny.
fs: PR-fix. Use ARRAY_SIZE macro

As suggested by @indutny.
fs: Improve performance of buildStatsObject.

Implement @trevnorris's suggestion to store the buildStatsObject
function as an Eternal in the C++ layer. This should perform better than
getting the buildStatsObject function from the stats object using
stats->Get.