Make it easier to do a "crater" run of toml

[epage]

While conformity is the goal, we will sometimes need to be beholden to our biggest user, cargo, to support broken crates for a period of time (e.g. rust-lang/cargo#9932)

To help catch these earlier, we should make a tool, stored in the repo, for extracting all of the toml files and parsing them.

See https://github.com/ehuss/crates-scanner for a starting point. The crates-index crate could help.