auto-bisect regressions based on the pr-by-pr builds

[nikomatsakis]

Since we now have builds available from every PR, when cargobomb detects a regression, it could in principle bisect to find the precise PR that caused it. This would be amazing.

[Mark-Simulacrum]

Feel free to steal or call out to code from https://github.com/Mark-Simulacrum/bisect-rust.