benchmark: add benches for fs.stat & fs.statSync

Add very simple benchmarks for `fs.stat` and `fs.statSync` as
well as `fs.lstat` and `fs.lstatSync` based on the `readdir`
benchmarks.

PR-URL: https://github.com/nodejs/node/pull/8338


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>