crev-to-vet converter that converts a proof db into audits.toml

[kornelski]

cc #632

Here's an example file:

https://github.com/kornelski/crev-proofs/blob/master/audits.toml

cargo install cargo-vet
cargo vet init
cargo vet import kornelski https://raw.githubusercontent.com/kornelski/crev-proofs/master/audits.toml
cargo vet