Allow more-easily-parsable output format for libtest benchmark harness

[frewsxcv]

I thought about creating a utility to visualize the results of cargo bench. Has there been any thought about adding a --format=json flag to cargo bench so this would be easier? I might be interested in implementing this if this gets approval

[nagisa]

Has been a bunch of RFCs on the topic already, latest being rust-lang/rfcs#1284.

[Mark-Simulacrum]

I think this would indeed need an RFC; closing.