process: improve memoryUsage() performance

Creating an object in JS and using a typed array to transfer values
from C++ to JS is faster than creating an object and setting
properties in C++.

The included benchmark shows ~34% increase in performance with this
change.

PR-URL: https://github.com/nodejs/node/pull/11497


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>