The prost RUSTSEC

[dan-fritchman]

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.