Port rollup unrolling from rustc-perf to bors #285

Currently, the rust-timer bot performs unrolling of rollups. It takes each PR from the rollup, builds it on its own, and creates artifacts that can then be benchmarked. This is implemented here.

Because the unrolling is not done by the main merge bot (bors), it needs a separate set of branches (perf-tmp and try-perf), which brings additional churn and infra complexity. We should port this unrolling to bors, since it is its responsibility to handle rollups, run CI builds, etc.