Make crater test better incr comp

[spastorino]

We were discussing on Zulip about some simple ideas on how to test incr compilation better.

Maybe it could be nice to build crate versions N - 1 and then incrementally build crate versions N to better exercise incr comp code.

[pietroalbini]

This might get messy, as Crater runs every build in a shared target directory. Not sure how much it would skew the results.

[michaelwoerister]

For reference, during the initial development of incremental compilation we had the cargo-incremental tool that did something similar. The main problem I remember was that we tested large parts of a given crate's commit history and many commits were written against much older versions of Rust. But if you only test a few versions back that shouldn't be so much of a problem.