Putting bors on a PIP

[Eh2406]

I was independently coming up with similar ideas for automatic roll ups at:

Add support for optimistic concurrency testing

But in the discussion of:

Proposal: move to bors-ng

The described batching algorithm does seem like it has a high potential to not work well for Rust, with our long turnaround times, and high intermittent rates. I’d be more comfortable adopting it if it supported our existing workflow and let us gradually experiment with others. If bors-ng is going to be maintained, and it can support our workflow, and somebody is willing to do the work of the transition, and preferably if it can remain under the venerable ‘bors’ account, then it might be worth …

2 things seemed to be made clearer.

1. brson, and presumably others would need convincing before accepting the automatic roll ups.
2. There was interest in switching to bors-ng, that already has an implementation this stufe.