Add a dedicated benchmark slave

[brson]

@huonw has a tool ready to go to report performance metrics but the inconsistency of our build slaves makes the numbers useless.

[reem]

Triage: @huonw any progress on this tool?

[huonw]

The build slave hasn't been set up (I don't have control over this).

[alexcrichton]

Closing in favor of rust-lang/rfcs#635