The `prost` RUSTSEC

`cargo audit` runs such as this one:  
https://github.com/dan-fritchman/Layout21/pull/35/checks?check_run_id=10213135939

Have been turning up this RUSTSEC:  
https://rustsec.org/advisories/RUSTSEC-2021-0073.html

Seems a quick upgrade to `prost` should cover it.  
While these checks are being run in Layout21, the change will actually need to be in https://github.com/vlsir/vlsir. 