Add a simple JSON.dump and JSON.load benchmark

[eregon]

• Restoring benchmarks/ohai.json from dad9fbfbfbfcfe743563348f2c17aedecfabe803.

The commit intended to fix #117, that's still fixed as these new files are not included in the gem s.files =.

[eregon]

@hsbt Could you review this too? (if not, who should I ask?)

[hsbt]

👍